fix: ensure private app is registered in sys.modules after loading

app.py

@@ -120,6 +120,7 @@ def _load_private_app_once():
             raise ImportError(f"Could not load spec from {app_file_path}")
         
         private_app = importlib.util.module_from_spec(spec)
+        sys.modules[spec.name] = private_app
         spec.loader.exec_module(private_app)
         # If the private app exposes a thumbnail generator, run it once
         # so example images are ready even when we bypass its main().