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/cache.py

@@ -14,12 +14,13 @@ from pydantic import BaseModel
 
 from artdag_common import render
 from artdag_common.middleware import wants_html, wants_json
+from artdag_common.middleware.auth import UserContext
 
 from ..dependencies import (
     require_auth, get_templates, get_redis_client,
     get_cache_manager, get_current_user
 )
-from ..services.auth_service import UserContext, AuthService
+from ..services.auth_service import AuthService
 from ..services.cache_service import CacheService
 
 router = APIRouter()