refactor(marketing): 修改促销活动相关字段类型

- 将 price 字段从 Integer 改为 String，以适应更多种价格表示需求
- 将 canCoupon 字段默认值设为 false，提高数据一致性

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

@@ -36,13 +36,13 @@ public class Promotion {
 
     private String type;
 
-    private Integer price;
+    private String price;
 
     private Boolean isAllDay;
 
     private String dayTime;
 
-    private Boolean canCoupon;
+    private Boolean canCoupon = false;
 
     private String status;