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/dependencies.py

@@ -64,10 +64,15 @@ async def get_current_user(request: Request) -> Optional[UserContext]:
     # Try header first (API clients)
     ctx = get_user_from_header(request)
     if ctx:
+        # Add l2_server from settings
+        ctx.l2_server = settings.l2_server
         return ctx
 
     # Fall back to cookie (browser)
-    return get_user_from_cookie(request)
+    ctx = get_user_from_cookie(request)
+    if ctx:
+        ctx.l2_server = settings.l2_server
+    return ctx
 
 
 async def require_auth(request: Request) -> UserContext: