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.

.run/GatewayApplication.run.xml

@@ -1,21 +0,0 @@
-<component name="ProjectRunConfigurationManager">
-  <configuration default="false" name="GatewayApplication" type="SpringBootApplicationConfigurationType" factoryName="Spring Boot" nameIsGenerated="true">
-    <option name="envFilePaths">
-      <option value="$PROJECT_DIR$/gateway.env" />
-    </option>
-    <module name="gateway" />
-    <selectedOptions>
-      <option name="environmentVariables" />
-    </selectedOptions>
-    <option name="SPRING_BOOT_MAIN_CLASS" value="dev.rheinsw.gateway.GatewayApplication" />
-    <extension name="coverage">
-      <pattern>
-        <option name="PATTERN" value="dev.rheinsw.gateway.*" />
-        <option name="ENABLED" value="true" />
-      </pattern>
-    </extension>
-    <method v="2">
-      <option name="Make" enabled="true" />
-    </method>
-  </configuration>
-</component>