Phase 10: Per-User Rate Limits — messages + tokens, quota UI, admin usage

Backend:
- max_ai_messages_per_day + max_ai_tokens_per_day on User model (nullable, override)
- Migration 008: add columns + seed default settings (100 msgs, 500K tokens)
- usage_service: count today's messages + tokens, check quota, get limits
- GET /chats/quota returns usage vs limits + reset time
- POST /chats/{id}/messages checks quota before streaming (429 if exceeded)
- Admin user schemas expose both limit fields
- GET /admin/usage returns per-user daily message + token counts
- admin_user_service allows updating both limit fields

Frontend:
- Chat header shows "X/Y messages · XK/YK tokens" with red highlight at limit
- Quota refreshes every 30s via TanStack Query
- Admin usage page with table: user, messages today, tokens today
- Route + sidebar entry for admin usage
- English + Russian translations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/admin_user_service.py

@@ -49,7 +49,7 @@ async def create_user(
 
 async def update_user(db: AsyncSession, user_id: uuid.UUID, **kwargs) -> User:
     user = await get_user(db, user_id)
-    allowed = {"role", "is_active", "max_chats", "full_name"}
+    allowed = {"role", "is_active", "max_chats", "max_ai_messages_per_day", "max_ai_tokens_per_day", "full_name"}
     for key, value in kwargs.items():
         if key in allowed and value is not None:
             setattr(user, key, value)