Pass email to UserContext from OAuth callback

The account token endpoint returns user.email as "username".
Store it explicitly so coop fragment middleware can use it.

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

app/routers/auth.py

@@ -140,11 +140,13 @@ async def callback(request: Request):
         return RedirectResponse(url="/", status_code=302)
 
     # Map OAuth response to artdag UserContext
+    # Note: account token endpoint returns user.email as "username"
     display_name = data.get("display_name", "")
     username = data.get("username", "")
+    email = username  # OAuth response "username" is the user's email
     actor_id = f"@{username}"
 
-    user = UserContext(username=username, actor_id=actor_id)
+    user = UserContext(username=username, actor_id=actor_id, email=email)
 
     response = RedirectResponse(url=next_url, status_code=302)
     set_auth_cookie(response, user)