Update dependencies and remove gateway

* Remove Gateway module, configuration, and references
* Update backend services to directly handle requests under `/api` prefix
* Adjust frontend contact route to connect directly to the server
* Centralize authentication logic and integrate token refresh mechanism

backend/common/src/main/java/dev/rheinsw/common/controller/exception/ApiException.java

@@ -0,0 +1,15 @@
+package dev.rheinsw.common.controller.exception;
+
+/**
+ * @author Thatsaphorn Atchariyaphap
+ * @since 06.07.25
+ */
+public class ApiException extends RuntimeException {
+    public ApiException(String message) {
+        super(message);
+    }
+
+    public ApiException(Throwable cause) {
+        super(cause);
+    }
+}

backend/common/src/main/java/dev/rheinsw/common/controller/exception/handler/GlobalExceptionHandler.java

@@ -0,0 +1,48 @@
+package dev.rheinsw.common.controller.exception.handler;
+
+import dev.rheinsw.common.controller.exception.ApiException;
+import dev.rheinsw.common.usecase.exception.UseCaseException;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ControllerAdvice;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+
+import java.time.Instant;
+import java.util.List;
+
+/**
+ * @author Thatsaphorn Atchariyaphap
+ * @since 06.07.25
+ */
+@ControllerAdvice
+public class GlobalExceptionHandler {
+
+    @ExceptionHandler(ApiException.class)
+    public ResponseEntity<ApiErrorResponse> handleBusinessException(ApiException ex) {
+        return ResponseEntity.badRequest().body(
+                new ApiErrorResponse(Instant.now(), ex.getMessage(), List.of(ex.getMessage()))
+        );
+    }
+
+    @ExceptionHandler(UseCaseException.class)
+    public ResponseEntity<ApiErrorResponse> handleUseCaseException(UseCaseException ex) {
+        return ResponseEntity.badRequest().body(
+                new ApiErrorResponse(Instant.now(), ex.getMessage(), List.of(ex.getMessage()))
+        );
+    }
+
+
+    @ExceptionHandler(Exception.class)
+    public ResponseEntity<ApiErrorResponse> handleGeneric(Exception ex) {
+        ex.printStackTrace(); // log the stack trace
+        return ResponseEntity.internalServerError().body(
+                new ApiErrorResponse(Instant.now(), "Ein unerwarteter Fehler ist aufgetreten", List.of(ex.getMessage()))
+        );
+    }
+
+    public record ApiErrorResponse(
+            Instant timestamp,
+            String message,
+            List<String> errors
+    ) {
+    }
+}

backend/common/src/main/java/dev/rheinsw/common/usecase/exception/UseCaseException.java

@@ -0,0 +1,15 @@
+package dev.rheinsw.common.usecase.exception;
+
+/**
+ * @author Thatsaphorn Atchariyaphap
+ * @since 06.07.25
+ */
+public class UseCaseException extends RuntimeException {
+    public UseCaseException(String message) {
+        super(message);
+    }
+
+    public UseCaseException(String message, Throwable cause) {
+        super(message, cause);
+    }
+}