chore: migrate backend to monorepo apps and biome

apps/gateway/src/gateway.controller.ts

@@ -0,0 +1,12 @@
+import { Controller, Get } from '@nestjs/common';
+import { GatewayService } from './gateway.service';
+
+@Controller()
+export class GatewayController {
+  constructor(private readonly gatewayService: GatewayService) {}
+
+  @Get()
+  getHello(): string {
+    return this.gatewayService.getHello();
+  }
+}