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/api/ai/retrieve/route.ts

@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { getStalwartCredentials } from '@/lib/stalwart/credentials';
 import { serverSearchMail, hydrateMailRefs } from '@/lib/ai/retrieval/mail-embeddings';
+import { configManager } from '@/lib/admin/config-manager';
 import { logger } from '@/lib/logger';
 
 export const runtime = 'nodejs';
@@ -31,6 +32,14 @@ export async function POST(request: NextRequest) {
     return new NextResponse(null, { status: 404 });
   }
 
+  // Real enforcement, not cosmetic client hiding (docs/ADMIN-AI-POLICY-CONSOLE-SPEC.md
+  // §6): an admin can disable mail-content-to-embeddings augmentation
+  // independent of disabling the `server` chat class outright.
+  await configManager.ensureLoaded();
+  if (!configManager.getAiConsoleConfig().retrievalEnabled) {
+    return NextResponse.json({ error: 'retrieval is disabled by admin policy' }, { status: 403 });
+  }
+
   let body: { query?: unknown; limit?: unknown };
   try {
     body = await request.json();