feat: 新增用户地址、收藏、商品评价和订单项模块

- 新增 UserAddress/UserFavorite/ProductReview/OrderItem 实体类
- 新增对应的 DTO、Repository、Service 和 Controller
- 新增 OrderMigrationService 订单数据迁移服务

src/main/java/com/org/flashsalesystem/service/OrderMigrationService.java

@@ -0,0 +1,63 @@
+package com.org.flashsalesystem.service;
+
+import com.org.flashsalesystem.dto.ProductDTO;
+import com.org.flashsalesystem.entity.Order;
+import com.org.flashsalesystem.entity.OrderItem;
+import com.org.flashsalesystem.repository.OrderItemRepository;
+import com.org.flashsalesystem.repository.OrderRepository;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+@Service
+@Slf4j
+public class OrderMigrationService {
+
+    @Autowired
+    private OrderRepository orderRepository;
+
+    @Autowired
+    private OrderItemRepository orderItemRepository;
+
+    @Autowired
+    private ProductService productService;
+
+    @Transactional
+    public Map<String, Object> migrateLegacyOrderItems() {
+        List<Order> orders = orderRepository.findAll();
+        int migrated = 0;
+        int skipped = 0;
+
+        for (Order order : orders) {
+            if (orderItemRepository.existsByOrderId(order.getId())) {
+                skipped++;
+                continue;
+            }
+
+            ProductDTO product = productService.getProductById(order.getProductId());
+            OrderItem orderItem = new OrderItem();
+            orderItem.setOrderId(order.getId());
+            orderItem.setProductId(order.getProductId());
+            orderItem.setProductName(product != null ? product.getName() : "未知商品");
+            orderItem.setProductImageUrl(product != null ? product.getImageUrl() : null);
+            orderItem.setPrice(order.getQuantity() != null && order.getQuantity() > 0
+                    ? order.getTotalPrice().divide(java.math.BigDecimal.valueOf(order.getQuantity()), 2, java.math.RoundingMode.HALF_UP)
+                    : order.getTotalPrice());
+            orderItem.setQuantity(order.getQuantity());
+            orderItem.setSubtotal(order.getTotalPrice());
+            orderItemRepository.save(orderItem);
+            migrated++;
+        }
+
+        Map<String, Object> result = new HashMap<>();
+        result.put("totalOrders", orders.size());
+        result.put("migrated", migrated);
+        result.put("skipped", skipped);
+        return result;
+    }
+}