Add sharing backend: shares CRUD + ACL enforcement everywhere (#16)

- domain: Garden gains a computed MyRole ("owner"/"editor"/"viewer"); new
  ShareWithUser (share + recipient identity); sentinels ErrShareUserNotFound,
  ErrCannotShareWithSelf, ErrShareExists.
- store/shares.go: GetShareRole, ListSharesForGarden (joined with users),
  CreateShare (UNIQUE → ErrShareExists), UpdateShareRole, DeleteShare.
- store/gardens.go: ListGardensForActor returns owned + shared-with-me gardens,
  each carrying my_role (replaces the owner-only list).
- service: requireGardenRole now consults garden_shares (owner implicit, else the
  share's role, else masked ErrNotFound) and stamps MyRole on every read-through.
  UpdateGarden tightened to OWNER-only (editors edit contents, not the garden).
  UpdatePlanting only re-checks plant visibility when the plant is CHANGED, so a
  shared editor can edit a plop that uses the owner's private plant.
- service/shares.go: ListShares/AddShare/UpdateShareRole (owner only), RemoveShare
  (owner or a recipient leaving). AddShare targets an existing account by exact
  email; unknown → 404, self → 400, duplicate → 409.
- api: GET,POST /gardens/:id/shares and PATCH,DELETE /gardens/:id/shares/:userId.

Tests: the full {owner, editor, viewer, stranger} × {read, mutate object, mutate
planting, edit garden, delete garden, manage shares} ACL matrix; shared gardens
carry my_role; AddShare error cases; role upgrade + self-leave; plus a two-user
HTTP flow. The matrix caught the editor-cannot-edit-owner's-plant bug above.

GOWORK=off go build/vet/test ./internal/... green.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/api/shares_test.go

@@ -0,0 +1,91 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"strconv"
+	"testing"
+)
+
+func sharesPath(gid int64) string { return "/api/v1/gardens/" + strconv.FormatInt(gid, 10) + "/shares" }
+func sharePath(gid, userID int64) string {
+	return sharesPath(gid) + "/" + strconv.FormatInt(userID, 10)
+}
+
+func TestSharingFlowAPI(t *testing.T) {
+	r := authEngine(t, localCfg())
+	a := registerAndCookie(t, r, "[email protected]")
+	b := registerAndCookie(t, r, "[email protected]")
+	gid := createGardenAPI(t, r, a, "A's yard")
+
+	// A bed to mutate later.
+	w := doJSON(t, r, http.MethodPost, objectsPath(gid),
+		map[string]any{"kind": "bed", "xCm": 500, "yCm": 500, "widthCm": 200, "heightCm": 200}, a)
+	oid := int64(decodeMap(t, w.Body.Bytes())["id"].(float64))
+
+	// Unknown email → 404 SHARE_USER_NOT_FOUND.
+	w = doJSON(t, r, http.MethodPost, sharesPath(gid), map[string]any{"email": "[email protected]", "role": "viewer"}, a)
+	if w.Code != http.StatusNotFound {
+		t.Fatalf("share unknown email = %d, want 404 (body %s)", w.Code, w.Body.String())
+	}
+
+	// Share with B as viewer → 201.
+	w = doJSON(t, r, http.MethodPost, sharesPath(gid), map[string]any{"email": "[email protected]", "role": "viewer"}, a)
+	if w.Code != http.StatusCreated {
+		t.Fatalf("share viewer = %d, body %s", w.Code, w.Body.String())
+	}
+	bID := int64(decodeMap(t, w.Body.Bytes())["userId"].(float64))
+
+	// B lists → the garden with myRole viewer.
+	w = doJSON(t, r, http.MethodGet, "/api/v1/gardens", nil, b)
+	var list []map[string]any
+	if err := json.Unmarshal(w.Body.Bytes(), &list); err != nil {
+		t.Fatalf("decode b list: %v", err)
+	}
+	if len(list) != 1 || list[0]["myRole"] != "viewer" {
+		t.Fatalf("b list = %+v, want one garden with myRole viewer", list)
+	}
+
+	// B reads /full (200) but can't mutate (403) or manage shares (403).
+	if w := doJSON(t, r, http.MethodGet, fullPath(gid), nil, b); w.Code != http.StatusOK {
+		t.Errorf("b read full = %d, want 200", w.Code)
+	}
+	if w := doJSON(t, r, http.MethodPatch, objectPath(oid), map[string]any{"xCm": 100, "version": 1}, b); w.Code != http.StatusForbidden {
+		t.Errorf("viewer mutate object = %d, want 403", w.Code)
+	}
+	if w := doJSON(t, r, http.MethodGet, sharesPath(gid), nil, b); w.Code != http.StatusForbidden {
+		t.Errorf("viewer list shares = %d, want 403", w.Code)
+	}
+
+	// A upgrades B to editor.
+	if w := doJSON(t, r, http.MethodPatch, sharePath(gid, bID), map[string]any{"role": "editor"}, a); w.Code != http.StatusOK {
+		t.Fatalf("upgrade role = %d, body %s", w.Code, w.Body.String())
+	}
+
+	// B can now move the object, but still can't rename the garden (owner-only).
+	if w := doJSON(t, r, http.MethodPatch, objectPath(oid), map[string]any{"xCm": 100, "version": 1}, b); w.Code != http.StatusOK {
+		t.Errorf("editor move object = %d, want 200 (body %s)", w.Code, w.Body.String())
+	}
+	if w := doJSON(t, r, http.MethodPatch, gardenPath(gid),
+		map[string]any{"name": "hijack", "widthCm": 100, "heightCm": 100, "unitPref": "metric", "version": 1}, b); w.Code != http.StatusForbidden {
+		t.Errorf("editor rename garden = %d, want 403", w.Code)
+	}
+
+	// A removes the share → B loses access.
+	if w := doJSON(t, r, http.MethodDelete, sharePath(gid, bID), nil, a); w.Code != http.StatusNoContent {
+		t.Fatalf("remove share = %d", w.Code)
+	}
+	if w := doJSON(t, r, http.MethodGet, fullPath(gid), nil, b); w.Code != http.StatusNotFound {
+		t.Errorf("b read after unshare = %d, want 404", w.Code)
+	}
+	if w := doJSON(t, r, http.MethodGet, "/api/v1/gardens", nil, b); w.Body.String() != "[]" {
+		t.Errorf("b list after unshare = %s, want []", w.Body.String())
+	}
+}
+
+func TestSharesRequireAuth(t *testing.T) {
+	r := authEngine(t, localCfg())
+	if w := doJSON(t, r, http.MethodGet, "/api/v1/gardens/1/shares", nil, nil); w.Code != http.StatusUnauthorized {
+		t.Errorf("unauthenticated list shares = %d, want 401", w.Code)
+	}
+}