feat(common): add ForbiddenException with 403 handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/com/recipeapp/common/ForbiddenException.java

@@ -0,0 +1,7 @@
+package com.recipeapp.common;
+
+public class ForbiddenException extends RuntimeException {
+    public ForbiddenException(String message) {
+        super(message);
+    }
+}

backend/src/main/java/com/recipeapp/common/GlobalExceptionHandler.java

@@ -32,6 +32,12 @@ public class GlobalExceptionHandler {
                 .body(ApiError.of("CONFLICT", ex.getMessage()));
     }
 
+    @ExceptionHandler(ForbiddenException.class)
+    public ResponseEntity<ApiError> handleForbidden(ForbiddenException ex) {
+        return ResponseEntity.status(HttpStatus.FORBIDDEN)
+                .body(ApiError.of("FORBIDDEN", ex.getMessage()));
+    }
+
     @ExceptionHandler(ValidationException.class)
     public ResponseEntity<ApiError> handleBusinessValidation(ValidationException ex) {
         return ResponseEntity.status(HttpStatus.UNPROCESSABLE_ENTITY)