refactor(vo): 重构VO对象结构，统一字段命名和接口实现

重构所有VO对象，统一字段命名规范，移除冗余字段，优化接口实现
新增Voable接口用于VO对象转换
调整BaseViewModel和ProjectBasedViewModel接口定义
更新相关服务和控制器以适应VO对象变更

client/src/main/java/com/ecep/contract/service/PurchaseOrderItemService.java

@@ -2,10 +2,10 @@ package com.ecep.contract.service;
 
 import org.springframework.stereotype.Service;
 
-import com.ecep.contract.model.PurchaseOrderItem;
 import com.ecep.contract.vm.PurchaseOrderItemViewModel;
+import com.ecep.contract.vo.PurchaseOrderItemVo;
 
 @Service
-public class PurchaseOrderItemService extends QueryService<PurchaseOrderItem, PurchaseOrderItemViewModel> {
+public class PurchaseOrderItemService extends QueryService<PurchaseOrderItemVo, PurchaseOrderItemViewModel> {
 
 }