Remove Gateway service and configuration

- Delete Gateway module, associated Spring Boot application, and related configuration (`GatewayApplication.java`, `application.yml`, and `pom.xml`).
- Remove Gateway references in `docker-compose.yml`, `.gitlab-ci.yml`, and `backend/pom.xml`.
- Update backend services to directly handle requests under `/api` prefix (e.g., `/api/customers`, `/api/contact`).
- Adjust frontend contact route to connect directly to the server, replacing gateway references with server URLs.

backend/pom.xml

@@ -11,7 +11,6 @@
 
     <modules>
         <module>common</module>
-        <module>gateway</module>
         <module>server</module>
     </modules>