feat(order): 添加倒计时字段

- 在 IsBuyBatchRespVO 类中添加 countdown 属性，初始值为 3600秒（1小时）
- 用于表示订单倒计时时间，增强用户体验

src/main/java/com/ycwl/basic/model/mobile/order/IsBuyBatchRespVO.java

@@ -17,6 +17,7 @@ public class IsBuyBatchRespVO {
     private BigDecimal origPrice = BigDecimal.ZERO;
     private Integer couponId;
     private Integer couponRecordId;
+    private Integer countdown = 3600;
     private BigDecimal couponPrice = BigDecimal.ZERO;
     private BigDecimal slashPrice;