refactor(marketing): 将优惠券有效日期类型从 LocalDateTime 改为 LocalDate

- 修改了 MercCoupon 类中的 validStartDate 和 validEndDate 字段类型
- 从 LocalDateTime改为 LocalDate，更符合实际业务需求

src/main/java/com/xynet/marketing/entity/MercCoupon.java

@@ -42,9 +42,9 @@ public class MercCoupon implements Serializable {
 
     private Integer validDay;
 
-    private LocalDateTime validStartDate;
+    private LocalDate validStartDate;
 
-    private LocalDateTime validEndDate;
+    private LocalDate validEndDate;
 
     private Integer useMinPrice;