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/signals.py

@@ -0,0 +1,35 @@
+from django.conf import settings
+from django.db.models.signals import post_save
+from django.dispatch import receiver
+
+
+@receiver(post_save, sender=settings.AUTH_USER_MODEL)
+def sync_dm_chat_identity(sender, instance, created, update_fields, **kwargs):
+    """Keep DM chat name/icon in sync when a user updates their username or avatar."""
+    if created:
+        return
+
+    changed = set(update_fields) if update_fields else None  # None = full save
+    username_changed = changed is None or 'username' in changed
+    avatar_changed = changed is None or 'avatar' in changed
+
+    if not (username_changed or avatar_changed):
+        return
+
+    from .models import Chat
+
+    dm_chats = Chat.objects.filter(
+        chat_type=Chat.ChatType.DM,
+        members=instance,
+    ).exclude(owner=instance)
+
+    if not dm_chats.exists():
+        return
+
+    update_kwargs = {}
+    if username_changed:
+        update_kwargs['name'] = instance.username
+    if avatar_changed:
+        update_kwargs['icon'] = instance.avatar.name if instance.avatar else None
+
+    dm_chats.update(**update_kwargs)