Add fallback for user avatar URL in notifications

Ensures that a default avatar URL is used if the user object lacks an avatar_url attribute or if an exception occurs, improving robustness of notification cover images.

app/models/notification.py

@@ -117,10 +117,14 @@ class ChannelMessageBase(NotificationDetail):
         receiver: list[int],
         channel_type: "ChannelType",
     ) -> Self:
+        try:
+            avatar_url = getattr(user, 'avatar_url', 'https://lazer-data.g0v0.top/default.jpg') or 'https://lazer-data.g0v0.top/default.jpg'
+        except Exception:
+            avatar_url = 'https://lazer-data.g0v0.top/default.jpg'
         instance = cls(
             title=truncate(message.content, CONTENT_TRUNCATE),
             type=channel_type.value.lower(),
-            cover_url=user.avatar_url,
+            cover_url=avatar_url,
         )
         instance._message = message
         instance._user = user