fix(pricing): 修复优惠券库存检查逻辑

- 修正totalQuantity为NULL或0时不限制总量的判断逻辑
- 优化claimedQuantity为空时的默认值处理
- 仅在totalQuantity大于0时更新已领取数量
- 完善MD文档中字段语义描述和配置示例
- 更新SQL表字段说明及典型配置组合示例

src/main/java/com/ycwl/basic/pricing/service/impl/CouponServiceImpl.java

@@ -231,8 +231,10 @@ public class CouponServiceImpl implements ICouponService {
             }
             
             // 5. 检查库存（如果有总量限制）
-            if (coupon.getTotalQuantity() != null && coupon.getClaimedQuantity() != null) {
-                if (coupon.getClaimedQuantity() >= coupon.getTotalQuantity()) {
+            // totalQuantity为NULL或0表示不限制总量
+            if (coupon.getTotalQuantity() != null && coupon.getTotalQuantity() > 0) {
+                int currentClaimed = (coupon.getClaimedQuantity() == null ? 0 : coupon.getClaimedQuantity());
+                if (currentClaimed >= coupon.getTotalQuantity()) {
                     return CouponClaimResult.failure(CouponClaimResult.ERROR_COUPON_OUT_OF_STOCK, "优惠券已领完");
                 }
             }
@@ -276,7 +278,8 @@ public class CouponServiceImpl implements ICouponService {
             }
 
             // 10. 更新优惠券已领取数量（区分于已使用数量）
-            if (coupon.getTotalQuantity() != null) {
+            // 仅在有总量限制时才更新claimedQuantity(totalQuantity为正整数)
+            if (coupon.getTotalQuantity() != null && coupon.getTotalQuantity() > 0) {
                 int updatedClaimedQuantity = (coupon.getClaimedQuantity() == null ? 0 : coupon.getClaimedQuantity()) + 1;
                 coupon.setClaimedQuantity(updatedClaimedQuantity);
                 couponConfigMapper.updateById(coupon);