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/agent/runtime_test.go

@@ -13,7 +13,6 @@ import (
 	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
 	"gitea.stevedudenhoeffer.com/steve/majordomo/provider/fake"
 
-	"gitea.stevedudenhoeffer.com/steve/pansy/internal/config"
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/service"
 )
@@ -232,24 +231,24 @@ func TestReadOnlyTurnWritesNoChangeSet(t *testing.T) {
 	}
 }
 
-// TestNewRunnerNeedsConfiguration — an instance with no key must not get a
-// half-built runner; the caller treats the error as "no assistant" and carries on.
+// TestNewRunnerNeedsConfiguration — an instance with no key or no model must not
+// get a half-built runner; the caller treats the error as "no assistant" and
+// carries on. (Whether the assistant is ENABLED is resolved before NewRunner is
+// reached, so it isn't NewRunner's concern any more.)
 func TestNewRunnerNeedsConfiguration(t *testing.T) {
 	svc, _ := newAgentTestService(t)
-	for _, cfg := range []*config.Config{
-		{Agent: config.AgentConfig{Enabled: false, OllamaCloudAPIKey: "k", Model: "ollama-cloud/x"}},
-		{Agent: config.AgentConfig{Enabled: true, OllamaCloudAPIKey: "", Model: "ollama-cloud/x"}},
-		{Agent: config.AgentConfig{Enabled: true, OllamaCloudAPIKey: "k", Model: ""}},
+	for _, tc := range []struct{ key, model string }{
+		{"", "ollama-cloud/x"},
+		{"k", ""},
+		{"k", "   "},
 	} {
-		if _, err := NewRunner(svc, cfg); err == nil {
-			t.Errorf("NewRunner accepted %+v", cfg.Agent)
+		if _, err := NewRunner(svc, tc.key, tc.model); err == nil {
+			t.Errorf("NewRunner accepted key=%q model=%q", tc.key, tc.model)
 		}
 	}
 	// A model spec naming a provider that doesn't exist is a configuration
 	// error, not a panic at first use.
-	if _, err := NewRunner(svc, &config.Config{Agent: config.AgentConfig{
-		Enabled: true, OllamaCloudAPIKey: "k", Model: "nonesuch/model",
-	}}); err == nil {
+	if _, err := NewRunner(svc, "k", "nonesuch/model"); err == nil {
 		t.Error("NewRunner accepted an unresolvable model spec")
 	}
 }