fix: add ValidationPipe

backend/src/main.ts

 import { NestFactory } from '@nestjs/core';
 import { ConfigService } from '@nestjs/config';
 import { AppModule } from './app.module';
+import { ValidationPipe } from '@nestjs/common';
 
 async function bootstrap(): Promise<void> {
   const app = await NestFactory.create(AppModule);
   const configService = app.get(ConfigService);
+  app.useGlobalPipes(
+    new ValidationPipe({
+      transform: true,
+    }),
+  );
 
   const port = configService.get<number>('PORT') | 4000;
   console.log(`NestJS server is listening on: http://localhost:${port}`);