diff --git a/internal/api/api.go b/internal/api/api.go
index be57af0..87ef607 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -44,6 +44,9 @@ func New(cfg *config.Config, svc *service.Service) *gin.Engine {
 	h := &handlers{cfg: cfg, svc: svc}
 
 	v1 := r.Group("/api/v1")
+	// CSRF defense for every state-changing API call (no-op unless PANSY_BASE_URL
+	// is set; see csrfGuard).
+	v1.Use(h.csrfGuard())
 	v1.GET("/healthz", healthz)
 
 	// Auth endpoints are exempt from requireAuth (you can't be logged in yet);
diff --git a/internal/api/auth.go b/internal/api/auth.go
index 69a7e0a..ca06dec 100644
--- a/internal/api/auth.go
+++ b/internal/api/auth.go
@@ -4,6 +4,7 @@ import (
 	"errors"
 	"log/slog"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 
@@ -30,7 +31,7 @@ type registerRequest struct {
 
 type loginRequest struct {
 	Email    string `json:"email" binding:"required,email"`
-	Password string `json:"password" binding:"required"`
+	Password string `json:"password" binding:"required,max=1024"`
 }
 
 // register creates a local account and logs it in (sets the session cookie).
@@ -47,15 +48,11 @@ func (h *handlers) register(c *gin.Context) {
 		Password:    req.Password,
 	})
 	if err != nil {
-		h.writeServiceError(c, err)
+		writeServiceError(c, err)
 		return
 	}
 
-	if err := h.startSession(c, user.ID); err != nil {
-		writeAPIError(c, http.StatusInternalServerError, "INTERNAL", "could not start session")
-		return
-	}
-	c.JSON(http.StatusOK, user)
+	h.startSessionAndRespond(c, user)
 }
 
 // login verifies credentials and sets the session cookie.
@@ -68,15 +65,11 @@ func (h *handlers) login(c *gin.Context) {
 
 	user, err := h.svc.Login(c.Request.Context(), req.Email, req.Password)
 	if err != nil {
-		h.writeServiceError(c, err)
+		writeServiceError(c, err)
 		return
 	}
 
-	if err := h.startSession(c, user.ID); err != nil {
-		writeAPIError(c, http.StatusInternalServerError, "INTERNAL", "could not start session")
-		return
-	}
-	c.JSON(http.StatusOK, user)
+	h.startSessionAndRespond(c, user)
 }
 
 // logout deletes the current session (if any) and clears the cookie. It is
@@ -102,37 +95,84 @@ func (h *handlers) providers(c *gin.Context) {
 }
 
 // requireAuth is middleware that rejects requests without a valid session and,
-// on success, stashes the resolved user in the context. Feature routers added by
-// later issues (gardens, objects, …) attach this to their protected groups.
+// on success, stashes the resolved user in the context and slides the cookie's
+// lifetime forward in step with the server-side session (otherwise an active
+// user would be logged out 30 days after login regardless of activity). Feature
+// routers added by later issues (gardens, objects, …) attach this to their
+// protected groups.
 func (h *handlers) requireAuth() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		token, err := c.Cookie(sessionCookie)
 		if err != nil || token == "" {
-			writeAPIError(c, http.StatusUnauthorized, "UNAUTHENTICATED", "authentication required")
-			c.Abort()
+			abortUnauthenticated(c)
 			return
 		}
-		user, err := h.svc.ResolveSession(c.Request.Context(), token)
+		user, expiresAt, err := h.svc.ResolveSession(c.Request.Context(), token)
 		if err != nil {
 			// Any resolution failure (missing/expired/tampered) is 401, never 404:
 			// the client should log in, not think a resource is gone.
-			writeAPIError(c, http.StatusUnauthorized, "UNAUTHENTICATED", "authentication required")
-			c.Abort()
+			abortUnauthenticated(c)
 			return
 		}
 		c.Set(actorKey, user)
+		// Refresh the client cookie to the session's current expiry so browser and
+		// server slide together.
+		h.setSessionCookie(c, token, expiresAt)
 		c.Next()
 	}
 }
 
-// startSession issues a session and writes the cookie.
-func (h *handlers) startSession(c *gin.Context, userID int64) error {
-	token, expiresAt, err := h.svc.CreateSession(c.Request.Context(), userID)
+// abortUnauthenticated writes the standard 401 and stops the handler chain.
+func abortUnauthenticated(c *gin.Context) {
+	writeAPIError(c, http.StatusUnauthorized, "UNAUTHENTICATED", "authentication required")
+	c.Abort()
+}
+
+// csrfGuard rejects state-changing requests whose Origin header doesn't match
+// the instance's own origin — a defense against login/logout CSRF, which
+// SameSite=Lax cookies alone do not prevent. It is a no-op unless PANSY_BASE_URL
+// is set (so it never interferes with the local Vite dev proxy), and it allows
+// requests with no Origin (non-browser clients like curl, which aren't a CSRF
+// vector). Safe methods are always allowed.
+func (h *handlers) csrfGuard() gin.HandlerFunc {
+	var wantHost string
+	if u, err := url.Parse(h.cfg.BaseURL); err == nil {
+		wantHost = u.Host
+	}
+	return func(c *gin.Context) {
+		if wantHost == "" {
+			c.Next()
+			return
+		}
+		switch c.Request.Method {
+		case http.MethodGet, http.MethodHead, http.MethodOptions:
+			c.Next()
+			return
+		}
+		if origin := c.GetHeader("Origin"); origin != "" {
+			if u, err := url.Parse(origin); err != nil || !strings.EqualFold(u.Host, wantHost) {
+				writeAPIError(c, http.StatusForbidden, "CROSS_ORIGIN", "cross-origin request rejected")
+				c.Abort()
+				return
+			}
+		}
+		c.Next()
+	}
+}
+
+// startSessionAndRespond issues a session, sets the cookie, and returns the user
+// as JSON — the shared tail of register and login.
+func (h *handlers) startSessionAndRespond(c *gin.Context, user *domain.User) {
+	token, expiresAt, err := h.svc.CreateSession(c.Request.Context(), user.ID)
 	if err != nil {
-		return err
+		// The account exists and is usable via login; only the auto-login cookie
+		// failed. Log it so the operator can see the underlying DB problem.
+		slog.Error("api: could not start session", "user_id", user.ID, "error", err)
+		writeAPIError(c, http.StatusInternalServerError, "INTERNAL", "could not start session")
+		return
 	}
 	h.setSessionCookie(c, token, expiresAt)
-	return nil
+	c.JSON(http.StatusOK, user)
 }
 
 // cookieSecure reports whether the session cookie should carry the Secure
@@ -143,6 +183,10 @@ func (h *handlers) cookieSecure() bool {
 }
 
 func (h *handlers) setSessionCookie(c *gin.Context, token string, expiresAt time.Time) {
+	// Callers only ever pass a future expiry (login/register set now+TTL;
+	// requireAuth uses a session already checked to be unexpired), so this is
+	// always well above the floor; the floor merely avoids emitting a delete
+	// cookie (MaxAge<=0) from a pathological clock skew.
 	maxAge := max(int(time.Until(expiresAt).Seconds()), 1)
 	c.SetSameSite(http.SameSiteLaxMode)
 	c.SetCookie(sessionCookie, token, maxAge, "/", "", h.cookieSecure(), true)
@@ -161,7 +205,7 @@ func mustActor(c *gin.Context) *domain.User {
 
 // writeServiceError maps a service-layer sentinel error to pansy's JSON error
 // envelope. Login failures never leak which of email/password was wrong.
-func (h *handlers) writeServiceError(c *gin.Context, err error) {
+func writeServiceError(c *gin.Context, err error) {
 	switch {
 	case errors.Is(err, domain.ErrInvalidCredentials):
 		writeAPIError(c, http.StatusUnauthorized, "INVALID_CREDENTIALS", "invalid email or password")
diff --git a/internal/api/auth_test.go b/internal/api/auth_test.go
index 60215d9..af0b338 100644
--- a/internal/api/auth_test.go
+++ b/internal/api/auth_test.go
@@ -35,16 +35,22 @@ func localCfg() *config.Config {
 	return &config.Config{Registration: config.RegistrationOpen, LocalAuth: true}
 }
 
-// doJSON issues a JSON request, optionally carrying a session cookie.
-func doJSON(t *testing.T, r *gin.Engine, method, path string, body any, cookie *http.Cookie) *httptest.ResponseRecorder {
+// encodeBody JSON-encodes v into a reader for a request body.
+func encodeBody(t *testing.T, v any) *bytes.Buffer {
 	t.Helper()
 	var buf bytes.Buffer
-	if body != nil {
-		if err := json.NewEncoder(&buf).Encode(body); err != nil {
+	if v != nil {
+		if err := json.NewEncoder(&buf).Encode(v); err != nil {
 			t.Fatalf("encode body: %v", err)
 		}
 	}
-	req := httptest.NewRequest(method, path, &buf)
+	return &buf
+}
+
+// doJSON issues a JSON request, optionally carrying a session cookie.
+func doJSON(t *testing.T, r *gin.Engine, method, path string, body any, cookie *http.Cookie) *httptest.ResponseRecorder {
+	t.Helper()
+	req := httptest.NewRequest(method, path, encodeBody(t, body))
 	req.Header.Set("Content-Type", "application/json")
 	if cookie != nil {
 		req.AddCookie(cookie)
@@ -153,6 +159,71 @@ func TestProvidersEndpoint(t *testing.T) {
 	}
 }
 
+func TestAuthenticatedRequestRefreshesCookie(t *testing.T) {
+	// requireAuth must re-set the session cookie so the browser's Max-Age slides
+	// with the server session rather than expiring 30 days after login.
+	r := authEngine(t, localCfg())
+	w := doJSON(t, r, http.MethodPost, "/api/v1/auth/register",
+		map[string]string{"email": "a@example.com", "displayName": "Alice", "password": "password123"}, nil)
+	cookie := sessionCookieFrom(t, w)
+
+	w = doJSON(t, r, http.MethodGet, "/api/v1/auth/me", nil, cookie)
+	if w.Code != http.StatusOK {
+		t.Fatalf("me status = %d", w.Code)
+	}
+	// A fresh session cookie should be present on the /me response.
+	refreshed := sessionCookieFrom(t, w)
+	if refreshed.Value != cookie.Value {
+		t.Errorf("refreshed cookie value changed: got %q want same token", refreshed.Value)
+	}
+	if refreshed.MaxAge <= 0 {
+		t.Errorf("refreshed cookie Max-Age = %d, want > 0", refreshed.MaxAge)
+	}
+}
+
+func TestCSRFGuardRejectsCrossOrigin(t *testing.T) {
+	cfg := localCfg()
+	cfg.BaseURL = "https://pansy.example.com"
+	r := authEngine(t, cfg)
+
+	body := map[string]string{"email": "a@example.com", "displayName": "Alice", "password": "password123"}
+
+	// Cross-origin POST is rejected before touching the service.
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/register", encodeBody(t, body))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Origin", "https://evil.example.com")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+	if w.Code != http.StatusForbidden {
+		t.Fatalf("cross-origin register status = %d, want 403", w.Code)
+	}
+
+	// Same-origin POST is allowed.
+	req = httptest.NewRequest(http.MethodPost, "/api/v1/auth/register", encodeBody(t, body))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Origin", "https://pansy.example.com")
+	w = httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+	if w.Code != http.StatusOK {
+		t.Fatalf("same-origin register status = %d, want 200 (body %s)", w.Code, w.Body.String())
+	}
+}
+
+func TestCSRFGuardNoopWithoutBaseURL(t *testing.T) {
+	// In local dev (no PANSY_BASE_URL) the guard must not interfere, even with a
+	// foreign Origin (the Vite dev proxy forwards the browser's origin).
+	r := authEngine(t, localCfg())
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/register",
+		encodeBody(t, map[string]string{"email": "a@example.com", "displayName": "Alice", "password": "password123"}))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Origin", "http://localhost:5173")
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, req)
+	if w.Code != http.StatusOK {
+		t.Fatalf("dev cross-origin register status = %d, want 200", w.Code)
+	}
+}
+
 func TestSecureCookieWithHTTPSBaseURL(t *testing.T) {
 	cfg := localCfg()
 	cfg.BaseURL = "https://pansy.example.com"
diff --git a/internal/domain/domain.go b/internal/domain/domain.go
index 7561146..57e4766 100644
--- a/internal/domain/domain.go
+++ b/internal/domain/domain.go
@@ -117,25 +117,25 @@ type GardenShare struct {
 // GardenObject is any placeable object in a garden (bed, container, tree, path…).
 // Positioned by center point + rotation about center, in garden space.
 type GardenObject struct {
-	ID          int64    `json:"id"`
-	GardenID    int64    `json:"gardenId"`
-	Kind        string   `json:"kind"`
-	Name        string   `json:"name"`
-	Shape       string   `json:"shape"`
-	Points      *string  `json:"points,omitempty"` // reserved: JSON for polygons
-	XCM         float64  `json:"xCm"`
-	YCM         float64  `json:"yCm"`
-	WidthCM     float64  `json:"widthCm"`
-	HeightCM    float64  `json:"heightCm"`
-	RotationDeg float64  `json:"rotationDeg"`
-	ZIndex      int      `json:"zIndex"`
-	Plantable   bool     `json:"plantable"`
-	Color       *string  `json:"color,omitempty"`
-	Props       *string  `json:"props,omitempty"` // kind-specific JSON
-	Notes       string   `json:"notes"`
-	Version     int64    `json:"version"`
-	CreatedAt   string   `json:"createdAt"`
-	UpdatedAt   string   `json:"updatedAt"`
+	ID          int64   `json:"id"`
+	GardenID    int64   `json:"gardenId"`
+	Kind        string  `json:"kind"`
+	Name        string  `json:"name"`
+	Shape       string  `json:"shape"`
+	Points      *string `json:"points,omitempty"` // reserved: JSON for polygons
+	XCM         float64 `json:"xCm"`
+	YCM         float64 `json:"yCm"`
+	WidthCM     float64 `json:"widthCm"`
+	HeightCM    float64 `json:"heightCm"`
+	RotationDeg float64 `json:"rotationDeg"`
+	ZIndex      int     `json:"zIndex"`
+	Plantable   bool    `json:"plantable"`
+	Color       *string `json:"color,omitempty"`
+	Props       *string `json:"props,omitempty"` // kind-specific JSON
+	Notes       string  `json:"notes"`
+	Version     int64   `json:"version"`
+	CreatedAt   string  `json:"createdAt"`
+	UpdatedAt   string  `json:"updatedAt"`
 }
 
 // Plant is a catalog entry. OwnerID nil means a read-only seeded built-in.
@@ -157,17 +157,17 @@ type Plant struct {
 // Planting ("plop") is a circular patch of one plant, positioned in its parent
 // object's local frame. Count nil means it is derived from area / spacing².
 type Planting struct {
-	ID        int64    `json:"id"`
-	ObjectID  int64    `json:"objectId"`
-	PlantID   int64    `json:"plantId"`
-	XCM       float64  `json:"xCm"`
-	YCM       float64  `json:"yCm"`
-	RadiusCM  float64  `json:"radiusCm"`
-	Count     *int     `json:"count,omitempty"` // nil = derived
-	Label     *string  `json:"label,omitempty"`
-	PlantedAt *string  `json:"plantedAt,omitempty"`
-	RemovedAt *string  `json:"removedAt,omitempty"`
-	Version   int64    `json:"version"`
-	CreatedAt string   `json:"createdAt"`
-	UpdatedAt string   `json:"updatedAt"`
+	ID        int64   `json:"id"`
+	ObjectID  int64   `json:"objectId"`
+	PlantID   int64   `json:"plantId"`
+	XCM       float64 `json:"xCm"`
+	YCM       float64 `json:"yCm"`
+	RadiusCM  float64 `json:"radiusCm"`
+	Count     *int    `json:"count,omitempty"` // nil = derived
+	Label     *string `json:"label,omitempty"`
+	PlantedAt *string `json:"plantedAt,omitempty"`
+	RemovedAt *string `json:"removedAt,omitempty"`
+	Version   int64   `json:"version"`
+	CreatedAt string  `json:"createdAt"`
+	UpdatedAt string  `json:"updatedAt"`
 }
diff --git a/internal/service/auth.go b/internal/service/auth.go
index c5da735..6d284b2 100644
--- a/internal/service/auth.go
+++ b/internal/service/auth.go
@@ -3,12 +3,17 @@ package service
 import (
 	"context"
 	"errors"
+	"log/slog"
 	"strings"
 	"time"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
 )
 
+// maxPasswordLen caps accepted password length. It bounds argon2 input and
+// request work, and sits far above any real password.
+const maxPasswordLen = 1024
+
 // RegisterInput is the payload for local self-service signup.
 type RegisterInput struct {
 	Email       string
@@ -46,27 +51,22 @@ func (s *Service) Register(ctx context.Context, in RegisterInput) (*domain.User,
 
 	email := normalizeEmail(in.Email)
 	displayName := strings.TrimSpace(in.DisplayName)
-	if email == "" || displayName == "" || in.Password == "" {
+	if email == "" || displayName == "" || in.Password == "" || len(in.Password) > maxPasswordLen {
 		return nil, domain.ErrInvalidInput
 	}
 
-	count, err := s.store.CountUsers(ctx)
-	if err != nil {
-		return nil, err
-	}
-	// Closed registration still allows the very first account so a locked-down
-	// instance can be bootstrapped without editing config.
-	if count > 0 && !s.cfg.RegistrationOpen() {
-		return nil, domain.ErrRegistrationClosed
-	}
-
-	// Friendly duplicate check. The UNIQUE index is the real guard against a
-	// race; that path surfaces as a generic insert error (500), acceptable at
-	// household scale.
-	if _, err := s.store.GetUserByEmail(ctx, email); err == nil {
-		return nil, domain.ErrEmailTaken
-	} else if !errors.Is(err, domain.ErrNotFound) {
-		return nil, err
+	// Cheap best-effort gate so a closed instance doesn't burn argon2 work on
+	// signups it will reject anyway. The authoritative, race-free gate — plus
+	// atomic first-user-is-admin assignment and duplicate-email detection — lives
+	// in store.CreateUser's single INSERT.
+	if !s.cfg.RegistrationOpen() {
+		n, err := s.store.CountUsers(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if n > 0 {
+			return nil, domain.ErrRegistrationClosed
+		}
 	}
 
 	hash, err := hashPassword(in.Password)
@@ -78,8 +78,7 @@ func (s *Service) Register(ctx context.Context, in RegisterInput) (*domain.User,
 		Email:        email,
 		DisplayName:  displayName,
 		PasswordHash: &hash,
-		IsAdmin:      count == 0,
-	})
+	}, s.cfg.RegistrationOpen())
 }
 
 // Login verifies an email/password pair and returns the user. Unknown-email and
@@ -89,6 +88,10 @@ func (s *Service) Login(ctx context.Context, email, password string) (*domain.Us
 	if !s.cfg.LocalAuth {
 		return nil, domain.ErrLocalAuthDisabled
 	}
+	if len(password) > maxPasswordLen {
+		// No stored password is this long; reject without spending argon2 work.
+		return nil, domain.ErrInvalidCredentials
+	}
 
 	u, err := s.store.GetUserByEmail(ctx, normalizeEmail(email))
 	switch {
@@ -105,7 +108,13 @@ func (s *Service) Login(ctx context.Context, email, password string) (*domain.Us
 	}
 
 	ok, err := verifyPassword(*u.PasswordHash, password)
-	if err != nil || !ok {
+	if err != nil {
+		// A stored hash we can't parse is a data problem, not a wrong password;
+		// surface it so a corrupt row doesn't silently lock a user out unnoticed.
+		slog.Error("service: malformed stored password hash", "user_id", u.ID, "error", err)
+		return nil, domain.ErrInvalidCredentials
+	}
+	if !ok {
 		return nil, domain.ErrInvalidCredentials
 	}
 	return u, nil
@@ -129,38 +138,53 @@ func (s *Service) CreateSession(ctx context.Context, userID int64) (token string
 	return raw, exp, nil
 }
 
-// ResolveSession validates a raw bearer token and returns its user. An expired
-// session is deleted and treated as absent (domain.ErrNotFound). A still-valid
-// session has its expiry slid forward, but only when that moves it by more than
-// an hour, so a busy client doesn't write on every request.
-func (s *Service) ResolveSession(ctx context.Context, rawToken string) (*domain.User, error) {
+// ResolveSession validates a raw bearer token and returns its user and the
+// session's current expiry (so the caller can slide the client cookie in
+// lockstep with the server). An expired session is deleted and treated as absent
+// (domain.ErrNotFound). A still-valid session has its expiry slid forward, but
+// only when that moves it by more than an hour, so a busy client doesn't write
+// on every request.
+func (s *Service) ResolveSession(ctx context.Context, rawToken string) (*domain.User, time.Time, error) {
 	if rawToken == "" {
-		return nil, domain.ErrNotFound
+		return nil, time.Time{}, domain.ErrNotFound
 	}
 	hash := hashToken(rawToken)
 	sess, err := s.store.GetSession(ctx, hash)
 	if err != nil {
-		return nil, err
+		return nil, time.Time{}, err
 	}
 
 	exp, err := parseTime(sess.ExpiresAt)
 	if err != nil {
 		// A corrupt expiry means we can't trust the session; drop it.
-		_ = s.store.DeleteSession(ctx, hash)
-		return nil, domain.ErrNotFound
+		if delErr := s.store.DeleteSession(ctx, hash); delErr != nil {
+			slog.Warn("service: deleting session with corrupt expiry", "error", delErr)
+		}
+		return nil, time.Time{}, domain.ErrNotFound
 	}
 
 	now := s.now()
 	if !now.Before(exp) {
-		_ = s.store.DeleteSession(ctx, hash)
-		return nil, domain.ErrNotFound
+		if delErr := s.store.DeleteSession(ctx, hash); delErr != nil {
+			slog.Warn("service: deleting expired session", "error", delErr)
+		}
+		return nil, time.Time{}, domain.ErrNotFound
 	}
 
 	if newExp := now.Add(sessionTTL); newExp.Sub(exp) > time.Hour {
-		_ = s.store.TouchSession(ctx, hash, formatTime(newExp))
+		if err := s.store.TouchSession(ctx, hash, formatTime(newExp)); err != nil {
+			// Non-fatal: the session is still valid at its current expiry.
+			slog.Warn("service: sliding session expiry", "error", err)
+		} else {
+			exp = newExp
+		}
 	}
 
-	return s.store.GetUserByID(ctx, sess.UserID)
+	user, err := s.store.GetUserByID(ctx, sess.UserID)
+	if err != nil {
+		return nil, time.Time{}, err
+	}
+	return user, exp, nil
 }
 
 // Logout deletes the session behind a raw bearer token. It is idempotent.
diff --git a/internal/service/auth_test.go b/internal/service/auth_test.go
index f583452..5a85bde 100644
--- a/internal/service/auth_test.go
+++ b/internal/service/auth_test.go
@@ -3,6 +3,7 @@ package service
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 	"time"
 
@@ -98,6 +99,20 @@ func TestRegistrationClosedAllowsBootstrapThenBlocks(t *testing.T) {
 	}
 }
 
+func TestRejectsOverlongPassword(t *testing.T) {
+	s := newTestService(t, openConfig())
+	long := strings.Repeat("a", maxPasswordLen+1)
+
+	if _, err := s.Register(context.Background(), RegisterInput{Email: "a@example.com", DisplayName: "A", Password: long}); !errors.Is(err, domain.ErrInvalidInput) {
+		t.Errorf("register overlong err = %v, want ErrInvalidInput", err)
+	}
+
+	mustRegister(t, s, "b@example.com", "Bob", "password123")
+	if _, err := s.Login(context.Background(), "b@example.com", long); !errors.Is(err, domain.ErrInvalidCredentials) {
+		t.Errorf("login overlong err = %v, want ErrInvalidCredentials", err)
+	}
+}
+
 func TestLocalAuthDisabledRejectsRegisterAndLogin(t *testing.T) {
 	cfg := openConfig()
 	cfg.LocalAuth = false
@@ -139,7 +154,7 @@ func TestLoginRejectsOIDCOnlyUser(t *testing.T) {
 	iss, sub := "https://idp.example", "subject-1"
 	if _, err := s.store.CreateUser(context.Background(), &domain.User{
 		Email: "oidc@example.com", DisplayName: "O", OIDCIssuer: &iss, OIDCSubject: &sub,
-	}); err != nil {
+	}, true); err != nil {
 		t.Fatalf("seed oidc user: %v", err)
 	}
 	if _, err := s.Login(context.Background(), "oidc@example.com", "anything"); !errors.Is(err, domain.ErrInvalidCredentials) {
@@ -156,29 +171,32 @@ func TestSessionLifecycle(t *testing.T) {
 		t.Fatalf("CreateSession: %v", err)
 	}
 
-	got, err := s.ResolveSession(context.Background(), token)
+	got, exp, err := s.ResolveSession(context.Background(), token)
 	if err != nil {
 		t.Fatalf("ResolveSession: %v", err)
 	}
 	if got.ID != u.ID {
 		t.Errorf("resolved user %d, want %d", got.ID, u.ID)
 	}
+	if !exp.After(s.now()) {
+		t.Errorf("resolved expiry %v is not in the future", exp)
+	}
 
 	// Logout invalidates it.
 	if err := s.Logout(context.Background(), token); err != nil {
 		t.Fatalf("Logout: %v", err)
 	}
-	if _, err := s.ResolveSession(context.Background(), token); !errors.Is(err, domain.ErrNotFound) {
+	if _, _, err := s.ResolveSession(context.Background(), token); !errors.Is(err, domain.ErrNotFound) {
 		t.Errorf("resolve after logout err = %v, want ErrNotFound", err)
 	}
 }
 
 func TestResolveSessionRejectsGarbageToken(t *testing.T) {
 	s := newTestService(t, openConfig())
-	if _, err := s.ResolveSession(context.Background(), "not-a-real-token"); !errors.Is(err, domain.ErrNotFound) {
+	if _, _, err := s.ResolveSession(context.Background(), "not-a-real-token"); !errors.Is(err, domain.ErrNotFound) {
 		t.Errorf("garbage token err = %v, want ErrNotFound", err)
 	}
-	if _, err := s.ResolveSession(context.Background(), ""); !errors.Is(err, domain.ErrNotFound) {
+	if _, _, err := s.ResolveSession(context.Background(), ""); !errors.Is(err, domain.ErrNotFound) {
 		t.Errorf("empty token err = %v, want ErrNotFound", err)
 	}
 }
@@ -197,7 +215,7 @@ func TestSessionExpiryAndLazyDeletion(t *testing.T) {
 
 	// Jump past the 30-day TTL: the session must be treated as gone...
 	s.now = func() time.Time { return base.Add(sessionTTL + time.Hour) }
-	if _, err := s.ResolveSession(context.Background(), token); !errors.Is(err, domain.ErrNotFound) {
+	if _, _, err := s.ResolveSession(context.Background(), token); !errors.Is(err, domain.ErrNotFound) {
 		t.Fatalf("expired resolve err = %v, want ErrNotFound", err)
 	}
 	// ...and lazily deleted from the store.
@@ -219,9 +237,13 @@ func TestSessionSlidingRenewal(t *testing.T) {
 
 	// Use it 10 days later; expiry should slide forward.
 	s.now = func() time.Time { return base.Add(10 * 24 * time.Hour) }
-	if _, err := s.ResolveSession(context.Background(), token); err != nil {
+	_, resolvedExp, err := s.ResolveSession(context.Background(), token)
+	if err != nil {
 		t.Fatalf("ResolveSession: %v", err)
 	}
+	if !resolvedExp.After(firstExp) {
+		t.Errorf("returned expiry did not slide: %v not after %v", resolvedExp, firstExp)
+	}
 	sess, err := s.store.GetSession(context.Background(), hashToken(token))
 	if err != nil {
 		t.Fatalf("GetSession: %v", err)
diff --git a/internal/service/password.go b/internal/service/password.go
index fa34ae6..634b554 100644
--- a/internal/service/password.go
+++ b/internal/service/password.go
@@ -11,13 +11,13 @@ import (
 	"golang.org/x/crypto/argon2"
 )
 
-// argon2id parameters. ~64 MiB memory / 1 pass / 4 lanes is the interactive
-// profile recommended by the argon2 authors and is comfortable on a
-// self-hosted box. They are encoded into every stored hash, so raising them
-// later leaves old hashes verifiable.
+// argon2id parameters: RFC 9106's second recommended profile (m=64 MiB, t=3,
+// p=4) — the memory-constrained option, appropriate for a self-hosted box where
+// the 2 GiB first profile is too heavy. They are encoded into every stored hash,
+// so raising them later leaves old hashes verifiable.
 const (
 	argonMemKiB  = 64 * 1024 // 64 MiB
-	argonTime    = 1
+	argonTime    = 3
 	argonThreads = 4
 	argonKeyLen  = 32
 	argonSaltLen = 16
@@ -25,12 +25,27 @@ const (
 
 // errBadHash marks a stored hash string that could not be parsed — a data or
 // programming error, not a wrong password. Callers treat it as an auth failure
-// but should log it.
+// and log it (Login does).
 var errBadHash = errors.New("service: malformed password hash")
 
 // b64 is the padding-free base64 used inside the PHC-style hash string.
 var b64 = base64.RawStdEncoding
 
+// timingHash is a valid argon2id hash used to equalize login response time when
+// an email is unknown (see Service.dummyHash). It uses a fixed salt rather than
+// crypto/rand so it can never fail to be produced at startup — an all-important
+// property, since a missing timing hash would silently re-open account
+// enumeration. It guards nothing, so a static salt is safe; deriving it from the
+// live argon parameters keeps its cost matched to a real verify.
+func timingHash() string {
+	salt := []byte("pansy-timing-eq!") // exactly argonSaltLen (16) bytes
+	key := argon2.IDKey([]byte("x"), salt, argonTime, argonMemKiB, argonThreads, argonKeyLen)
+	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
+		argon2.Version, argonMemKiB, argonTime, argonThreads,
+		b64.EncodeToString(salt), b64.EncodeToString(key),
+	)
+}
+
 // hashPassword returns a self-describing argon2id hash in the PHC string format
 // "$argon2id$v=19$m=...,t=...,p=...$salt$hash" (all base64, no padding).
 func hashPassword(password string) (string, error) {
@@ -57,26 +72,31 @@ func verifyPassword(encoded, password string) (bool, error) {
 }
 
 // decodeHash parses a PHC-format argon2id string into its parameters, salt, and
-// derived key.
+// derived key. Any structural problem returns errBadHash (with zero values) so
+// the caller never has to distinguish parse failures.
 func decodeHash(encoded string) (mem, t uint32, threads uint8, salt, key []byte, err error) {
-	parts := strings.Split(encoded, "$")
-	// ["", "argon2id", "v=19", "m=..,t=..,p=..", "<salt>", "<hash>"]
-	if len(parts) != 6 || parts[1] != "argon2id" {
+	fail := func() (uint32, uint32, uint8, []byte, []byte, error) {
 		return 0, 0, 0, nil, nil, errBadHash
 	}
 
-	var version int
-	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil || version != argon2.Version {
-		return 0, 0, 0, nil, nil, errBadHash
+	parts := strings.Split(encoded, "$")
+	// ["", "argon2id", "v=19", "m=..,t=..,p=..", "<salt>", "<hash>"]
+	if len(parts) != 6 || parts[1] != "argon2id" {
+		return fail()
 	}
-	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &mem, &t, &threads); err != nil {
-		return 0, 0, 0, nil, nil, errBadHash
+
+	var version int
+	if _, e := fmt.Sscanf(parts[2], "v=%d", &version); e != nil || version != argon2.Version {
+		return fail()
+	}
+	if _, e := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &mem, &t, &threads); e != nil {
+		return fail()
 	}
 	if salt, err = b64.DecodeString(parts[4]); err != nil {
-		return 0, 0, 0, nil, nil, errBadHash
+		return fail()
 	}
 	if key, err = b64.DecodeString(parts[5]); err != nil || len(key) == 0 {
-		return 0, 0, 0, nil, nil, errBadHash
+		return fail()
 	}
 	return mem, t, threads, salt, key, nil
 }
diff --git a/internal/service/password_test.go b/internal/service/password_test.go
index 8470555..8e6b486 100644
--- a/internal/service/password_test.go
+++ b/internal/service/password_test.go
@@ -45,7 +45,7 @@ func TestVerifyPasswordRejectsMalformedHash(t *testing.T) {
 		"not-a-hash",
 		"$argon2id$v=19$m=65536,t=1,p=4$onlyfourparts",
 		"$argon2i$v=19$m=65536,t=1,p=4$c2FsdA$aGFzaA", // wrong variant
-		"$argon2id$v=1$m=65536,t=1,p=4$c2FsdA$aGFzaA",  // wrong version
+		"$argon2id$v=1$m=65536,t=1,p=4$c2FsdA$aGFzaA", // wrong version
 	} {
 		if _, err := verifyPassword(bad, "whatever"); err == nil {
 			t.Errorf("verifyPassword(%q) err = nil, want errBadHash", bad)
diff --git a/internal/service/service.go b/internal/service/service.go
index bf6aadc..b33cedd 100644
--- a/internal/service/service.go
+++ b/internal/service/service.go
@@ -1,9 +1,11 @@
-// Package service is pansy's business-logic seam: every operation is a method on
-// *Service taking (ctx, actor, args), and all permission checks and invariants
-// live here rather than in the HTTP handlers. REST handlers (internal/api) and,
-// later, agent tools (internal/agent) are thin adapters over these methods, so
-// both inherit the same rules. This file holds the shared plumbing; feature
-// methods live alongside it (auth.go, and gardens/objects/… in later issues).
+// Package service is pansy's business-logic seam: all permission checks and
+// invariants live here rather than in the HTTP handlers. Resource operations
+// take (ctx, actor, args) so every rule is enforced regardless of caller; the
+// auth operations here are the exception — they establish the actor, so they
+// take credentials rather than one. REST handlers (internal/api) and, later,
+// agent tools (internal/agent) are thin adapters over these methods, so both
+// inherit the same rules. This file holds the shared plumbing; feature methods
+// live alongside it (auth.go, and gardens/objects/… in later issues).
 package service
 
 import (
@@ -12,7 +14,6 @@ import (
 	"encoding/base64"
 	"encoding/hex"
 	"fmt"
-	"log/slog"
 	"time"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/config"
@@ -33,23 +34,21 @@ type Service struct {
 	cfg   *config.Config
 	// now is the clock, injectable so tests can advance time (session expiry).
 	now func() time.Time
-	// dummyHash is a valid argon2id hash of a throwaway password. Login verifies
-	// against it when an email is unknown so the response time doesn't reveal
-	// whether an account exists.
+	// dummyHash is a valid argon2id hash Login verifies against when an email is
+	// unknown, so response time doesn't reveal whether an account exists. It is
+	// produced by timingHash (fixed salt, no RNG) so it is always present — an
+	// empty one would silently re-open account enumeration.
 	dummyHash string
 }
 
-// New constructs a Service. It precomputes a dummy password hash used to
-// equalize login timing; if that fails (it shouldn't), login still works but
-// loses the timing defense.
+// New constructs a Service.
 func New(st *store.DB, cfg *config.Config) *Service {
-	s := &Service{store: st, cfg: cfg, now: time.Now}
-	if h, err := hashPassword("pansy-timing-equalizer-not-a-real-password"); err != nil {
-		slog.Warn("service: could not precompute login timing hash", "error", err)
-	} else {
-		s.dummyHash = h
+	return &Service{
+		store:     st,
+		cfg:       cfg,
+		now:       time.Now,
+		dummyHash: timingHash(),
 	}
-	return s
 }
 
 // formatTime renders a time as pansy's canonical UTC string.
diff --git a/internal/store/migrations/0002_sessions_expires_index.sql b/internal/store/migrations/0002_sessions_expires_index.sql
new file mode 100644
index 0000000..a956310
--- /dev/null
+++ b/internal/store/migrations/0002_sessions_expires_index.sql
@@ -0,0 +1,7 @@
+-- 0002_sessions_expires_index.sql — index sessions.expires_at.
+--
+-- The periodic sweep (and lazy cleanup) run `DELETE FROM sessions WHERE
+-- expires_at <= ?`; without this index that is a full table scan. The table is
+-- small at household scale, but the index is cheap insurance and keeps the sweep
+-- O(expired) rather than O(all sessions).
+CREATE INDEX idx_sessions_expires ON sessions (expires_at);
diff --git a/internal/store/store_test.go b/internal/store/store_test.go
index 9cd5154..9c5e4a3 100644
--- a/internal/store/store_test.go
+++ b/internal/store/store_test.go
@@ -42,15 +42,15 @@ func TestMigrateCreatesSchema(t *testing.T) {
 	if err := db.SQL().QueryRow(`SELECT max(version) FROM schema_migrations`).Scan(&version); err != nil {
 		t.Fatalf("read schema_migrations: %v", err)
 	}
-	if version != 1 {
-		t.Errorf("schema version = %d, want 1", version)
+	if version != 2 {
+		t.Errorf("schema version = %d, want 2", version)
 	}
 }
 
 func TestMigrateIsIdempotent(t *testing.T) {
 	db := openTestDB(t)
 
-	// A second Migrate must be a no-op and leave exactly one recorded version.
+	// A second Migrate must be a no-op and leave exactly one row per migration.
 	if err := db.Migrate(context.Background()); err != nil {
 		t.Fatalf("second Migrate: %v", err)
 	}
@@ -58,8 +58,8 @@ func TestMigrateIsIdempotent(t *testing.T) {
 	if err := db.SQL().QueryRow(`SELECT count(*) FROM schema_migrations`).Scan(&count); err != nil {
 		t.Fatalf("count schema_migrations: %v", err)
 	}
-	if count != 1 {
-		t.Errorf("schema_migrations rows = %d, want 1", count)
+	if count != 2 {
+		t.Errorf("schema_migrations rows = %d, want 2", count)
 	}
 }
 
diff --git a/internal/store/users.go b/internal/store/users.go
index e92a36b..a7e3548 100644
--- a/internal/store/users.go
+++ b/internal/store/users.go
@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"strings"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
 )
@@ -20,7 +21,7 @@ type scanner interface {
 
 // scanUser reads one users row. is_admin is stored as INTEGER 0/1 (the driver
 // returns it as int64, which does not convert straight to bool), so it is read
-// into an int and mapped.
+// into an int64 and mapped.
 func scanUser(s scanner) (*domain.User, error) {
 	var (
 		u       domain.User
@@ -39,15 +40,37 @@ func scanUser(s scanner) (*domain.User, error) {
 
 // CreateUser inserts a new user and returns the stored row (with generated id
 // and timestamps). PasswordHash/OIDCIssuer/OIDCSubject may be nil.
-func (d *DB) CreateUser(ctx context.Context, u *domain.User) (*domain.User, error) {
+//
+// Two invariants are enforced inside the single INSERT statement so concurrent
+// registrations can't violate them (SQLite serializes writers, and the COUNT
+// subqueries see the latest committed state):
+//   - is_admin is set iff this is the first user — no read-then-write window in
+//     which two "first" registrations both become admin.
+//   - the row is inserted only when the table is empty (bootstrap) or allowSignup
+//     is true; otherwise zero rows are affected and ErrRegistrationClosed is
+//     returned. This is the authoritative registration gate.
+//
+// A duplicate email trips the UNIQUE index and maps to ErrEmailTaken.
+func (d *DB) CreateUser(ctx context.Context, u *domain.User, allowSignup bool) (*domain.User, error) {
 	res, err := d.sql.ExecContext(ctx,
 		`INSERT INTO users (email, display_name, password_hash, oidc_issuer, oidc_subject, is_admin)
-		 VALUES (?, ?, ?, ?, ?, ?)`,
-		u.Email, u.DisplayName, u.PasswordHash, u.OIDCIssuer, u.OIDCSubject, boolToInt(u.IsAdmin),
+		 SELECT ?, ?, ?, ?, ?, (SELECT count(*) FROM users) = 0
+		 WHERE (SELECT count(*) FROM users) = 0 OR ?`,
+		u.Email, u.DisplayName, u.PasswordHash, u.OIDCIssuer, u.OIDCSubject, boolToInt(allowSignup),
 	)
 	if err != nil {
+		if isUniqueViolation(err) {
+			return nil, domain.ErrEmailTaken
+		}
 		return nil, fmt.Errorf("store: insert user: %w", err)
 	}
+	n, err := res.RowsAffected()
+	if err != nil {
+		return nil, fmt.Errorf("store: user insert rows: %w", err)
+	}
+	if n == 0 {
+		return nil, domain.ErrRegistrationClosed
+	}
 	id, err := res.LastInsertId()
 	if err != nil {
 		return nil, fmt.Errorf("store: user insert id: %w", err)
@@ -99,3 +122,10 @@ func boolToInt(b bool) int {
 	}
 	return 0
 }
+
+// isUniqueViolation reports whether err is a SQLite UNIQUE-constraint failure.
+// modernc surfaces these in the error text; the message is stable across SQLite
+// versions ("UNIQUE constraint failed: <table>.<column>").
+func isUniqueViolation(err error) bool {
+	return err != nil && strings.Contains(err.Error(), "UNIQUE constraint failed")
+}