fix: 修复文件上传接口的 CORS 跨域问题

在 FileUploadController 上添加 @CrossOrigin 注解
在 WebMvcConfig 中排除 /api/upload/** 路径的拦截器

backend/src/main/java/com/maternalmall/controller/FileUploadController.java

@@ -14,6 +14,7 @@ import java.util.UUID;
 
 @RestController
 @RequestMapping("/api")
+@CrossOrigin(origins = "*", allowedHeaders = "*", methods = {RequestMethod.GET, RequestMethod.POST, RequestMethod.PUT, RequestMethod.DELETE, RequestMethod.OPTIONS})
 public class FileUploadController {
 
     @Value("${app.upload-path:./uploads}")