config(cors): 更新跨域配置并应用到特定控制器

- 在 MercCouponController 和 PromotionController 中添加 @CrossOrigin 注解，允许所有来源
- 重构 WebConfig 类，修改跨域配置以允许所有来源

src/main/java/com/xynet/marketing/controller/merc/MercCouponController.java

@@ -25,6 +25,7 @@ import java.util.Map;
 @RequestMapping("merc/coupon")
 @AllArgsConstructor
 @Slf4j
+@CrossOrigin(origins = "*")
 public class MercCouponController {
     private final MercCouponService mercCouponService;
 

src/main/java/com/xynet/marketing/controller/merc/PromotionController.java

@@ -24,6 +24,7 @@ import java.util.Map;
 @RequestMapping("merc/promotion")
 @AllArgsConstructor
 @Slf4j
+@CrossOrigin(origins = "*")
 public class PromotionController {
 
     private PromotionService promotionService;