feat: enforce custom appearance policy

src/govoplan_access/backend/api/v1/auth.py

@@ -1013,7 +1013,8 @@ def update_profile(
             next_settings["ui"] = UserUiPreferences().model_dump()
         else:
             appearance = _effective_appearance_info(session, tenant=context.tenant, user=context.user)
-            stored_palette = _user_ui_preferences(context.user.settings).palette
+            stored_preferences = _user_ui_preferences(context.user.settings)
+            stored_palette = stored_preferences.palette
             if (
                 appearance.locked
                 and payload.ui_preferences.palette is not None
@@ -1023,6 +1024,17 @@ def update_profile(
                     status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
                     detail="The effective appearance policy locks the palette.",
                 )
+            requested_overrides = payload.ui_preferences.appearance_overrides
+            if (
+                "appearance_overrides" in payload.ui_preferences.model_fields_set
+                and not appearance.custom_overrides_allowed
+                and requested_overrides is not None
+                and requested_overrides != stored_preferences.appearance_overrides
+            ):
+                raise HTTPException(
+                    status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
+                    detail="The effective appearance policy does not allow personal custom overrides.",
+                )
             if (
                 payload.ui_preferences.navigation is not None
                 and payload.ui_preferences.navigation.locked