refactor: 后端核心模块功能增强与代码优化

- 完善 User/Product/Order 实体字段和关联关系
- 更新 DTO 适配新增字段
- 增强 Service 层业务逻辑和 Repository 查询方法
- 优化控制器接口，完善管理后台 API
- 新增请求监控过滤器和指标服务

src/main/java/com/org/flashsalesystem/entity/User.java

@@ -43,9 +43,15 @@ public class User {
     @Column(length = 20)
     private String phone;
 
+    @Column(length = 500)
+    private String avatar;
+
     @Column(name = "status", nullable = false)
     private Integer status = 1; // 1-正常, 0-禁用
 
+    @Column(name = "role", nullable = false, length = 20)
+    private String role = "USER";
+
     @Column(name = "last_login")
     private LocalDateTime lastLogin;
 
@@ -57,6 +63,9 @@ public class User {
 
     @PrePersist
     protected void onCreate() {
+        if (role == null || role.trim().isEmpty()) {
+            role = "USER";
+        }
         createdAt = LocalDateTime.now();
         updatedAt = LocalDateTime.now();
     }