feat(admin): build the AI Policy console (§6) — approved, spec now implemented

New admin tab "AI" (app/(main)/admin/_tabs/ai-policy.tsx): provider-class
toggles, server model allow-list, BYOK provider allow-list, seats/usage
(front-end for the already-real lib/ai/entitlement.ts), retrieval on/off,
consent text + version bump.

Real backend, not cosmetic: AiConsoleConfig persisted via config-manager
(lib/ai/types.ts, ai-policy.json in the CONFIG dir). New GET/PUT
/api/admin/ai/policy. Enforcement wired at every real chokepoint, not just
the picker: /api/ai/server/chat checks classesEnabled.server and the model
allow-list, /api/ai/retrieve checks retrievalEnabled, /api/ai/server/models
filters by allow-list. GET /api/ai/policy folds classesEnabled into the
classes list clients see.

Resolved the spec's 3 open questions as recommended: BYOK allow-list stays
client-side/advisory (wired into ai-assistant-settings.tsx's addProfile),
tier picker stays cosmetic, master aiAssistantEnabled toggle stays in the
existing Policy tab (this tab links to it instead of duplicating it).

Defaults preserve today's behavior exactly (classesEnabled/allowlists all
start empty/null) — turning this on changes nothing until an admin touches it.

app/(main)/admin/page.tsx

@@ -7,6 +7,7 @@ import { SettingsTab } from './_tabs/settings';
 import { BrandingTab } from './_tabs/branding';
 import { AuthTab } from './_tabs/auth';
 import { PolicyTab } from './_tabs/policy';
+import { AiPolicyTab } from './_tabs/ai-policy';
 import { PluginsTab } from './_tabs/plugins';
 import { ThemesTab } from './_tabs/themes';
 import { MarketplaceTab } from './_tabs/marketplace';
@@ -39,6 +40,7 @@ export default function AdminPage() {
     case 'branding': return <BrandingTab />;
     case 'auth': return <AuthTab />;
     case 'policy': return <PolicyTab />;
+    case 'ai-policy': return <AiPolicyTab />;
     case 'plugins': return <PluginsTab />;
     case 'themes': return <ThemesTab />;
     case 'marketplace': return <MarketplaceTab />;