✨(backend) persist user preferences for room defaults on the User model

Add attributes on the User model to persist per-user preferences for
the default link access level and the default room configuration.

The frontend will let users update these preferences and then reuse
them when generating a link through the webapp.

Persisting them on the backend (rather than in application memory
only) ensures the preferences survive across sessions and devices.

src/backend/core/api/serializers.py

@@ -31,9 +31,28 @@ class UserSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.User
-        fields = ["id", "email", "full_name", "short_name", "timezone", "language"]
+        fields = [
+            "id",
+            "email",
+            "full_name",
+            "short_name",
+            "timezone",
+            "language",
+            "default_room_access_level",
+            "default_room_configuration",
+        ]
         read_only_fields = ["id", "email", "full_name", "short_name"]
 
+    def validate_default_room_configuration(self, value):
+        """Validate the default room configuration against the RoomConfiguration schema."""
+        if value is None or value == {}:
+            return value
+        try:
+            RoomConfiguration.model_validate(value)
+        except PydanticValidationError as e:
+            raise serializers.ValidationError(e.errors()) from e
+        return value
+
 
 class UserLightSerializer(serializers.ModelSerializer):
     """Serialize users with limited fields."""