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/store/instance_settings.go

@@ -0,0 +1,81 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+
+	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
+)
+
+// The instance_settings row is seeded by migration 0010 and there is exactly one
+// (CHECK id = 1), so reads never branch on existence and writes never insert.
+
+const instanceSettingsColumns = `agent_model, agent_enabled, version, updated_at`
+
+// scanInstanceSettings reads the single settings row. agent_enabled is a nullable
+// INTEGER (NULL = inherit env), so it is scanned through sql.NullInt64.
+func scanInstanceSettings(s scanner) (*domain.InstanceSettings, error) {
+	var (
+		out     domain.InstanceSettings
+		enabled sql.NullInt64
+	)
+	if err := s.Scan(&out.AgentModel, &enabled, &out.Version, &out.UpdatedAt); err != nil {
+		return nil, err
+	}
+	if enabled.Valid {
+		b := enabled.Int64 != 0
+		out.AgentEnabled = &b
+	}
+	return &out, nil
+}
+
+// GetInstanceSettings returns the single settings row.
+func (d *DB) GetInstanceSettings(ctx context.Context) (*domain.InstanceSettings, error) {
+	s, err := scanInstanceSettings(d.sql.QueryRowContext(ctx,
+		`SELECT `+instanceSettingsColumns+` FROM instance_settings WHERE id = 1`))
+	if errors.Is(err, sql.ErrNoRows) {
+		// The migration seeds this row, so its absence is a broken database, not a
+		// normal "not found" the caller should paper over.
+		return nil, fmt.Errorf("store: instance_settings row missing (migration 0010 not applied?)")
+	}
+	if err != nil {
+		return nil, fmt.Errorf("store: get instance settings: %w", err)
+	}
+	return s, nil
+}
+
+// UpdateInstanceSettings applies a version-guarded update to the single row,
+// following the same optimistic-concurrency contract as every mutable resource:
+// the updated row on success, (current row, ErrVersionConflict) on a version
+// mismatch. There is no ErrNotFound path — the row always exists.
+//
+// agentEnabled is nil to store SQL NULL (inherit env), or a pointer to store an
+// explicit 0/1.
+func (d *DB) UpdateInstanceSettings(ctx context.Context, s *domain.InstanceSettings) (*domain.InstanceSettings, error) {
+	var enabled any
+	if s.AgentEnabled != nil {
+		enabled = boolToInt(*s.AgentEnabled)
+	}
+	updated, err := scanInstanceSettings(d.sql.QueryRowContext(ctx,
+		`UPDATE instance_settings
+		    SET agent_model = ?, agent_enabled = ?,
+		        version = version + 1,
+		        updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
+		  WHERE id = 1 AND version = ?
+		  RETURNING `+instanceSettingsColumns,
+		s.AgentModel, enabled, s.Version,
+	))
+	if errors.Is(err, sql.ErrNoRows) {
+		current, gerr := d.GetInstanceSettings(ctx)
+		if gerr != nil {
+			return nil, gerr
+		}
+		return current, domain.ErrVersionConflict
+	}
+	if err != nil {
+		return nil, fmt.Errorf("store: update instance settings: %w", err)
+	}
+	return updated, nil
+}