chore: migrate backend to monorepo apps and biome

apps/gateway/test/app.e2e-spec.ts

@@ -0,0 +1,21 @@
+import { INestApplication } from '@nestjs/common';
+import { Test, TestingModule } from '@nestjs/testing';
+import request from 'supertest';
+import { GatewayModule } from './../src/gateway.module';
+
+describe('GatewayController (e2e)', () => {
+  let app: INestApplication;
+
+  beforeEach(async () => {
+    const moduleFixture: TestingModule = await Test.createTestingModule({
+      imports: [GatewayModule],
+    }).compile();
+
+    app = moduleFixture.createNestApplication();
+    await app.init();
+  });
+
+  it('/ (GET)', () => {
+    return request(app.getHttpServer()).get('/').expect(200).expect('Hello World!');
+  });
+});