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.

docker-compose.yml

@@ -1,18 +1,11 @@
 services:
-  gateway:
-    image: registry.boomlab.party/rheinsw/rheinsw-mono-repo/gateway
-    container_name: gateway
-    env_file:
-      - ./gateway.env
-    restart: on-failure
-    networks:
-      - rheinsw-net
-
   server:
     image: registry.boomlab.party/rheinsw/rheinsw-mono-repo/server
     container_name: server
     env_file:
       - ./server.env
+    ports:
+      - "8080:8080"
     restart: on-failure
     networks:
       - rheinsw-net