feat: use simpler nitro instead of nestjs to implement mock service

apps/backend-mock/error.ts

@@ -0,0 +1,7 @@
+import type { NitroErrorHandler } from 'nitropack';
+
+const errorHandler: NitroErrorHandler = function (error, event) {
+  event.res.end(`[error handler] ${error.stack}`);
+};
+
+export default errorHandler;