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/api/v1/admin.py

@@ -218,3 +218,32 @@ async def preview_pdf_template(
 ):
     html = await pdf_template_service.render_preview(data.html_content)
     return PreviewResponse(html=html)
+
+
+# --- Usage Stats ---
+
+@router.get("/usage")
+async def get_usage_stats(
+    _admin: Annotated[User, Depends(require_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+):
+    from sqlalchemy import select, func
+    from app.models.user import User as UserModel
+    from app.services.usage_service import get_user_message_count_today, get_user_token_count_today, get_user_daily_limits
+
+    result = await db.execute(select(UserModel).where(UserModel.is_active == True).order_by(UserModel.username))  # noqa: E712
+    users = result.scalars().all()
+
+    stats = []
+    for u in users:
+        limits = await get_user_daily_limits(db, u)
+        stats.append({
+            "user_id": str(u.id),
+            "username": u.username,
+            "messages_today": await get_user_message_count_today(db, u.id),
+            "message_limit": limits["message_limit"],
+            "tokens_today": await get_user_token_count_today(db, u.id),
+            "token_limit": limits["token_limit"],
+        })
+
+    return {"users": stats}