修复 StatServiceImpl 中 totalSeconds 改为 Long 后的类型转换

avgOrderDuration 赋值处缺少 int 转型，导致编译错误：
long → Integer 不兼容。在除法结果上补充 (int) 转换。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

car-wash-service/src/main/java/com/kym/service/impl/StatServiceImpl.java

@@ -131,7 +131,7 @@ public class StatServiceImpl implements StatService {
        var avgOrderPrice = todayOrders.isEmpty() ? 0 : (int)todayIncome/todayOrders.size();
        
         // 平均洗车时长（秒）
-       var avgOrderDuration = todayOrders.isEmpty() ? 0L : todayOrders.stream().mapToLong(WashOrder::getTotalSeconds).sum() / todayOrders.size();
+       var avgOrderDuration = todayOrders.isEmpty() ? 0 : (int) (todayOrders.stream().mapToLong(WashOrder::getTotalSeconds).sum() / todayOrders.size());
 
         // 月度数据
         var monthOrders = washOrderService.lambdaQuery()