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/.gitlab-ci.yml

@@ -7,7 +7,6 @@ build_backend:
   artifacts:
     paths:
       - backend/common/target/
-      - backend/gateway/target/
       - backend/discovery/target/
       - backend/server/target
     expire_in: 1 hour
@@ -21,18 +20,6 @@ docker_common:
   needs:
     - build_backend
 
-docker_gateway:
-  extends: .docker-build-template
-  variables:
-    IMAGE_NAME: gateway
-    COMMON_IMAGE: "$CI_REGISTRY/$CI_PROJECT_PATH/common"
-    WORKDIR_PATH: backend
-    DOCKERFILE_PATH: Dockerfile.app
-    BUILD_FOLDER: "gateway/target"
-    MAIN_CLASS: dev.rheinsw.gateway.GatewayApplication
-  needs:
-    - build_backend
-    - docker_common
 
 docker_server:
   extends: .docker-build-template