Use UserContext from artdag_common, remove duplicate

- Updated requirements.txt to use art-common@11aa056 with l2_server field
- All routers now import UserContext from artdag_common
- Removed duplicate UserContext from auth_service.py
- dependencies.py sets l2_server from settings on user context

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routers/api.py

@@ -15,8 +15,8 @@ import yaml
 from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
 
+from artdag_common.middleware.auth import UserContext
 from ..dependencies import require_auth, get_redis_client, get_cache_manager
-from ..services.auth_service import UserContext
 
 router = APIRouter()
 logger = logging.getLogger(__name__)