Sync DM chat name/icon on user changes

Add a signal to keep direct-message chat name and icon in sync when a user updates their username or avatar, and register it in AppConfig.ready. Also initialize a DM chat's name/icon on creation from the other member's username/avatar if those fields are missing, saving only the changed fields.

backend/social/chat/views.py

@@ -53,7 +53,19 @@ class ChatViewSet(viewsets.ModelViewSet):
         return Chat.objects.filter(Q(members=user) | Q(owner=user)).distinct()
 
     def perform_create(self, serializer):
-        serializer.save(owner=self.request.user)
+        chat = serializer.save(owner=self.request.user)
+        if chat.chat_type == Chat.ChatType.DM:
+            other = chat.members.exclude(pk=self.request.user.pk).first()
+            if other:
+                update_fields = []
+                if not chat.name:
+                    chat.name = other.username
+                    update_fields.append('name')
+                if not chat.icon and other.avatar:
+                    chat.icon = other.avatar
+                    update_fields.append('icon')
+                if update_fields:
+                    chat.save(update_fields=update_fields)
 
     # ------------------------------------------------------------------
     # Member management