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/services/auth_service.py

@@ -6,10 +6,10 @@ import hashlib
 import base64
 import json
 from typing import Optional
-from dataclasses import dataclass
 
 import httpx
 
+from artdag_common.middleware.auth import UserContext
 from ..config import settings
 
 
@@ -21,15 +21,6 @@ REVOKED_KEY_PREFIX = "artdag:revoked:"
 USER_TOKENS_PREFIX = "artdag:user_tokens:"
 
 
-@dataclass
-class UserContext:
-    """User context from token."""
-    username: str
-    actor_id: str
-    token: Optional[str] = None
-    l2_server: Optional[str] = None
-
-
 class AuthService:
     """Service for authentication and token management."""