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/domain/domain.go

@@ -242,6 +242,22 @@ const (
 	ConflictUnsupported = "unsupported"
 )
 
+// InstanceSettings is the single row of instance-wide, admin-editable
+// configuration (#79). Secrets are deliberately absent — see migration 0010.
+//
+// Both agent fields express "inherit from the environment unless set":
+// AgentModel == "" falls back to PANSY_AGENT_MODEL; AgentEnabled == nil inherits
+// the env default. EffectiveAgent resolves them against the environment.
+type InstanceSettings struct {
+	// AgentModel overrides PANSY_AGENT_MODEL when non-empty. Passed verbatim to
+	// majordomo.Parse, exactly like the env var it shadows.
+	AgentModel string `json:"agentModel"`
+	// AgentEnabled overrides PANSY_AGENT_ENABLED when non-nil. nil = inherit.
+	AgentEnabled *bool  `json:"agentEnabled"`
+	Version      int64  `json:"version"`
+	UpdatedAt    string `json:"updatedAt"`
+}
+
 // User is a pansy account. It may have a local password, OIDC identity, or both.
 type User struct {
 	ID           int64   `json:"id"`