Admin-gated Settings: runtime model selection, is_admin enforced (#90)

Closes #79. Agent model + on/off move into an admin-only Settings section, and
is_admin is enforced for the first time. The live Runner is hot-swapped behind an
atomic.Pointer with routes always registered, so a settings change takes effect
with no restart; /capabilities reads the pointer. Secrets stay in the env, never
the DB. Precedence: Settings → env → default. Verified live: swap on/off/model,
bad spec → 400, race-clean. Gadfly blocking round addressed (dead code removed,
error-swallowing fixed, frontend 503 handling, holder dedup).

DESIGN.md

@@ -9,8 +9,9 @@ Work is tracked in Gitea issues; the tracking epic links every piece in dependen
 - **Placement model:** freeform plops (not a square-foot grid), scaled by real plant spacing. Grid snapping may come later as a toggle.
 - **Spacing is a plant-to-plant rule, so bed edges get half of it.** A bed edge is not a competitor for soil, light or water, so the outer row owes it half the spacing rather than a full one. `FillRegion` centres its lattice accordingly, and lets a plop — a *clump* three spacings across — cross the edge by up to half a spacing so its outermost plants land at that half-spacing. The rule, the square-foot-chart arithmetic behind it, and the failure mode it prevents are written out once in `hexCenters`; #75 is what getting it wrong looked like.
 - **Stack:** Go 1.26.x backend, module `gitea.stevedudenhoeffer.com/steve/pansy`; React + TypeScript + Vite + Tailwind frontend, production build embedded via `embed.FS` → one static binary (`CGO_ENABLED=0`).
-- **Users:** multi-user with ownership. Users own gardens; a garden can be shared with other users as viewer (read) or editor (edit content). Owner additionally shares/deletes.
+- **Users:** multi-user with ownership. Users own gardens; a garden can be shared with other users as viewer (read) or editor (edit content). Owner additionally shares/deletes. The first registered user is `is_admin` (set race-free inside the INSERT); admin gates instance-wide Settings (`requireAdmin`), the only thing that reads that flag.
 - **Auth:** OIDC-first (Authentik is the primary IdP), local argon2id passwords as an optional fallback.
+- **Instance settings (#79):** admin-editable, instance-wide config in a single-row `instance_settings` table — pansy's first DB-stored *instance* state (everything else hangs off a garden/object). Today it holds the agent model + on/off; **secrets never move here** — `OLLAMA_CLOUD_API_KEY` stays in the env so it doesn't land in backups or the undo history. Precedence: Settings value → env → default. The live agent Runner sits behind an `atomic.Pointer` in the API layer (`agentHolder`) with its routes always registered, so a settings change swaps it with no restart and no race against in-flight requests; `/capabilities` reads that pointer, so it reports what's live rather than what was configured at boot. The model/registry knowledge lives in one leaf package (`internal/agentmodel`) that both the runner and the settings validator import — agent imports service, so it can live in neither.
 - **Agentic future:** integration with majordomo/executus via typed Go tools (`llm.DefineTool[Args]`) wrapping the same service layer the REST API uses — not MCP/OpenAPI.
 
 ## Domain model
@@ -72,7 +73,8 @@ GET,POST /gardens/:id/journal PATCH,DELETE /journal/:id           (editor writes
 GET  /gardens/:id/journal/counts  ← entries per object, for the "has notes" indicator
 POST /agent/chat              ← SSE: step events, then the finished turn (editor only)
 GET,DELETE /gardens/:id/agent/history                             (the actor's own thread)
-GET  /capabilities            ← what this instance can do, so the UI offers only what works
+GET  /capabilities            ← what this instance can do RIGHT NOW (tracks the live agent, not just config)
+GET,PATCH /settings           ← instance-wide config (admin only): agent model + on/off
 GET,POST /gardens/:id/shares  PATCH,DELETE /gardens/:id/shares/:userId    (invite by email)
 GET,POST,DELETE /gardens/:id/share-link   ← the public read-only token for this garden
 GET  /public/gardens/:token   ← UNAUTHENTICATED read-only /full; the token is the capability