feat: 删除JSP视图层，完善评价和通知系统，新增拼团模块

- 删除所有 JSP 页面（20个文件），前端完全迁移至 Vue 3 SPA
- 完善评价系统：ReviewDialog 组件、用户评价历史页、评价状态检查API
- 新增通知系统：Notification 实体/仓库/服务/控制器，NotificationCenter 接入真实API
- 新增拼团模块：GroupBuying 全套后端和前端页面
- 修复 review check API 参数双重包装导致请求格式错误
- 修复通知 API 路径缺少 /api 前缀和响应格式处理
- MessageListenerService 集成 NotificationService 创建持久化通知

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

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

@@ -0,0 +1,98 @@
+package com.org.flashsalesystem.entity;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+import javax.persistence.*;
+import java.math.BigDecimal;
+import java.time.LocalDateTime;
+
+@Entity
+@Table(name = "group_buying")
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+public class GroupBuying {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Column(name = "product_id", nullable = false)
+    private Long productId;
+
+    @Column(name = "group_price", nullable = false, precision = 10, scale = 2)
+    private BigDecimal groupPrice;
+
+    @Column(name = "required_members", nullable = false)
+    private Integer requiredMembers = 2;
+
+    @Column(name = "duration_minutes", nullable = false)
+    private Integer durationMinutes = 1440;
+
+    @Column(name = "total_stock", nullable = false)
+    private Integer totalStock;
+
+    @Column(name = "remaining_stock", nullable = false)
+    private Integer remainingStock;
+
+    @Column(name = "max_per_user", nullable = false)
+    private Integer maxPerUser = 1;
+
+    /**
+     * 状态：0-草稿 1-未开始 2-进行中 3-已结束
+     */
+    @Column(nullable = false)
+    private Integer status = 0;
+
+    @Column(name = "start_time", nullable = false)
+    private LocalDateTime startTime;
+
+    @Column(name = "end_time", nullable = false)
+    private LocalDateTime endTime;
+
+    @Column(name = "created_at", nullable = false, updatable = false)
+    private LocalDateTime createdAt;
+
+    @Column(name = "updated_at")
+    private LocalDateTime updatedAt;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "product_id", insertable = false, updatable = false)
+    private Product product;
+
+    @PrePersist
+    protected void onCreate() {
+        createdAt = LocalDateTime.now();
+        updatedAt = LocalDateTime.now();
+    }
+
+    @PreUpdate
+    protected void onUpdate() {
+        updatedAt = LocalDateTime.now();
+    }
+
+    public boolean isActive() {
+        LocalDateTime now = LocalDateTime.now();
+        return now.isAfter(startTime) && now.isBefore(endTime) && status == 2;
+    }
+
+    public enum GroupBuyingStatus {
+        DRAFT(0, "草稿"),
+        PENDING(1, "未开始"),
+        ACTIVE(2, "进行中"),
+        ENDED(3, "已结束");
+
+        private final int code;
+        private final String description;
+
+        GroupBuyingStatus(int code, String description) {
+            this.code = code;
+            this.description = description;
+        }
+
+        public int getCode() { return code; }
+        public String getDescription() { return description; }
+    }
+}