Admin-gated Settings: runtime model selection, enforce is_admin (#79)

Moves the agent model out of env-only config into an admin-editable Settings
section, and enforces is_admin for the first time — it has been in the schema
since migration 0001, plumbed to the client, and checked nowhere.

Backend:
- Migration 0010: instance_settings, a single-row (CHECK id=1) table — pansy's
  first instance-level state. Holds agent_model ('' = inherit env) and
  agent_enabled (NULL = inherit env), version-guarded like every mutable row.
  SECRETS STAY IN ENV: OLLAMA_CLOUD_API_KEY is never stored here.
- requireAdmin at the service seam (authoritative) plus a cheap middleware
  early-403. Non-admin gets 403, not 404 — settings existence isn't masked.
- EffectiveAgent resolves DB-over-env (model, enabled); key always from env.
- The live Runner is hot-swapped, not built once. agentHolder holds it behind
  an atomic.Pointer; the chat routes are now registered UNCONDITIONALLY and
  nil-check agent.get(), so a settings change turns the assistant on/off/onto a
  new model with no restart and no race against in-flight readers. /capabilities
  reads the pointer, so it reports what's live, not what booted.
- internal/agentmodel is a new leaf package holding the one place that knows how
  to turn a spec into a model. Both agent (to run) and service (to validate a
  spec before storing it) import it; it can't live in agent, which imports
  service. Settings PATCH validates the spec via Parse, so a typo is a 400 now
  rather than a broken assistant on the next turn.

Frontend:
- /settings route (admin guard), a Settings page (model field, tri-state
  enabled, live status), nav link shown only to admins.
- useCapabilities drops staleTime:Infinity — the assistant can now change under
  a running page — and the settings save invalidates it.

Contract change: chat routes always exist, so "assistant off" is a runtime 503
+ capabilities:false, not a missing route. Updated the test that asserted the
old shape.

Verified live against the built binary: disable flips capabilities to false and
logs it; re-enable with a new model swaps it back; a bad spec is rejected 400;
the setting persists across a restart. Swap is race-clean under `go test -race`.

Docs: README (precedence + key-stays-in-env), DESIGN (decision + routes),
CLAUDE (don't re-add conditional route registration; key never in the DB).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/service/instance_settings.go

@@ -0,0 +1,110 @@
+package service
+
+import (
+	"context"
+	"strings"
+
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/agentmodel"
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
+)
+
+// Instance settings (#79): admin-editable, instance-wide configuration. The
+// admin gate lives HERE, in the seam, not in the handler — the same rule every
+// other permission follows. The API's requireAdmin middleware is a cheap early
+// 403, not the authority.
+
+// requireAdmin returns nil iff the actor is an admin, else ErrForbidden.
+//
+// ErrForbidden, not ErrNotFound: settings are not a resource whose existence is
+// masked. A logged-in non-admin knows the instance has settings; they simply may
+// not touch them. (Contrast objects/lots, where no-access masks existence.)
+func (s *Service) requireAdmin(ctx context.Context, actorID int64) error {
+	u, err := s.store.GetUserByID(ctx, actorID)
+	if err != nil {
+		return err
+	}
+	if !u.IsAdmin {
+		return domain.ErrForbidden
+	}
+	return nil
+}
+
+// GetInstanceSettings returns the instance settings for an admin.
+func (s *Service) GetInstanceSettings(ctx context.Context, actorID int64) (*domain.InstanceSettings, error) {
+	if err := s.requireAdmin(ctx, actorID); err != nil {
+		return nil, err
+	}
+	return s.store.GetInstanceSettings(ctx)
+}
+
+// InstanceSettingsPatch is a full replacement of the editable fields plus the
+// current version. Both agent fields carry their "inherit" sentinel: an empty
+// AgentModel means fall back to env, a nil AgentEnabled means inherit.
+type InstanceSettingsPatch struct {
+	AgentModel   string
+	AgentEnabled *bool
+	Version      int64
+}
+
+// UpdateInstanceSettings applies an admin's change, version-guarded. It returns
+// (current row, ErrVersionConflict) on a stale version, like every mutable
+// resource. The model spec is validated before it is stored, so a typo is a 400
+// now rather than a broken assistant on the next turn.
+//
+// It does NOT rebuild the running agent — that is the API layer's job, because
+// the live Runner lives there. The caller rebuilds on success.
+func (s *Service) UpdateInstanceSettings(ctx context.Context, actorID int64, patch InstanceSettingsPatch) (*domain.InstanceSettings, error) {
+	if err := s.requireAdmin(ctx, actorID); err != nil {
+		return nil, err
+	}
+	model := strings.TrimSpace(patch.AgentModel)
+	// Validate a non-empty spec up front. An empty one is the "inherit env"
+	// sentinel and needs no check — the env value was validated at boot.
+	if model != "" {
+		if err := agentmodel.Validate(s.cfg.Agent.OllamaCloudAPIKey, model); err != nil {
+			return nil, domain.ErrInvalidInput
+		}
+	}
+	return s.store.UpdateInstanceSettings(ctx, &domain.InstanceSettings{
+		AgentModel:   model,
+		AgentEnabled: patch.AgentEnabled,
+		Version:      patch.Version,
+	})
+}
+
+// EffectiveAgent resolves the agent configuration actually in force: DB settings
+// override the environment, and the API key always comes from the environment.
+//
+// This is internal plumbing (the API layer calls it to build the Runner), NOT an
+// admin-gated operation — resolving what's configured is not the same as editing
+// it, and the agent bootstrap must work before any user is even authenticated.
+type EffectiveAgent struct {
+	Model   string
+	Enabled bool
+	APIKey  string
+}
+
+// Ready mirrors config.AgentConfig.Ready: enabled, with a key and a model.
+func (e EffectiveAgent) Ready() bool {
+	return e.Enabled && e.APIKey != "" && e.Model != ""
+}
+
+// EffectiveAgent reads the settings row and layers it over the environment.
+func (s *Service) EffectiveAgent(ctx context.Context) (EffectiveAgent, error) {
+	st, err := s.store.GetInstanceSettings(ctx)
+	if err != nil {
+		return EffectiveAgent{}, err
+	}
+	eff := EffectiveAgent{
+		Model:   s.cfg.Agent.Model,
+		Enabled: s.cfg.Agent.Enabled,
+		APIKey:  s.cfg.Agent.OllamaCloudAPIKey,
+	}
+	if st.AgentModel != "" {
+		eff.Model = st.AgentModel
+	}
+	if st.AgentEnabled != nil {
+		eff.Enabled = *st.AgentEnabled
+	}
+	return eff, nil
+}