refactor: 移除后端跨域配置，由前端代理处理

- 移除 FileUploadController 的 @CrossOrigin 注解
- 恢复 WebMvcConfig 拦截器配置

backend/src/main/java/com/maternalmall/config/WebMvcConfig.java

@@ -20,10 +20,8 @@ public class WebMvcConfig implements WebMvcConfigurer {
 
     @Override
     public void addInterceptors(InterceptorRegistry registry) {
-        registry.addInterceptor(authInterceptor)
-                .addPathPatterns("/**")
-                .excludePathPatterns("/uploads/**")
-                .excludePathPatterns("/api/upload/**");
+        registry.addInterceptor(authInterceptor).addPathPatterns("/**")
+                .excludePathPatterns("/uploads/**");
     }
 
     @Override

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

@@ -14,7 +14,6 @@ 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}")