Fix actor_id: use email not display name

actor_id must match DB records — always use username (email).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/routers/auth.py

@@ -142,7 +142,7 @@ async def callback(request: Request):
     # Map OAuth response to artdag UserContext
     display_name = data.get("display_name", "")
     username = data.get("username", "")
-    actor_id = f"@{display_name}" if display_name else f"@{username}"
+    actor_id = f"@{username}"
 
     user = UserContext(username=username, actor_id=actor_id)