Add l2_server field to UserContext

Allows L1 servers to track which L2 server the user authenticated with.

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

artdag_common/middleware/auth.py

@@ -20,6 +20,7 @@ class UserContext:
     username: str
     actor_id: str  # Full actor ID like "@user@server.com"
     token: Optional[str] = None
+    l2_server: Optional[str] = None  # L2 server URL for this user
 
     @property
     def display_name(self) -> str: