refactor(asset): 优化保养项目设计，使用折中方案

变更内容：
1. 删除 maintainItemId 字段（冗余，无实际作用）
2. 保留 maintainItem 字段（直接存储项目名称）
3. 新增接口：获取已使用的保养项目列表（去重）
4. 前端可使用该接口提供下拉提示，提升用户体验
5. 添加 maintainItem 字段的非空校验

设计思路：
- 不引入保养项目字典表（避免过度设计）
- 通过 DISTINCT 查询提供已使用项目列表
- 用户可以输入新项目，也可以从已有项目中选择
- 后续如需规范化，可平滑升级到字典表方案

yudao-module-asset/yudao-module-asset-server/src/main/java/cn/iocoder/yudao/module/asset/controller/admin/vehiclemodel/VehicleModelController.java

@@ -104,4 +104,11 @@ public class VehicleModelController {
         return success(BeanUtils.toBean(list, VehicleModelRespVO.class));
     }
 
+    @GetMapping("/maintain-item/used-list")
+    @Operation(summary = "获取已使用的保养项目列表", description = "用于前端下拉提示，返回去重后的保养项目名称")
+    public CommonResult<List<String>> getUsedMaintainItems() {
+        List<String> list = vehicleModelService.getUsedMaintainItems();
+        return success(list);
+    }
+
 }

yudao-module-asset/yudao-module-asset-server/src/main/java/cn/iocoder/yudao/module/asset/controller/admin/vehiclemodel/vo/VehicleModelMaintainItemVO.java

@@ -5,6 +5,7 @@ import lombok.Data;
 
 import jakarta.validation.constraints.DecimalMin;
 import jakarta.validation.constraints.Min;
+import jakarta.validation.constraints.NotBlank;
 import java.math.BigDecimal;
 
 /**
@@ -19,10 +20,8 @@ public class VehicleModelMaintainItemVO {
     @Schema(description = "主键", example = "1")
     private Long id;
 
-    @Schema(description = "维保项目ID", example = "1")
-    private Long maintainItemId;
-
-    @Schema(description = "保养项目", example = "更换机油")
+    @Schema(description = "保养项目", requiredMode = Schema.RequiredMode.REQUIRED, example = "机油更换")
+    @NotBlank(message = "保养项目不能为空")
     private String maintainItem;
 
     @Schema(description = "保养内容", example = "更换机油、机滤")

yudao-module-asset/yudao-module-asset-server/src/main/java/cn/iocoder/yudao/module/asset/dal/dataobject/vehiclemodel/VehicleModelMaintainItemDO.java

@@ -9,7 +9,7 @@ import lombok.*;
 import java.math.BigDecimal;
 
 /**
- * 车型维保项目关联 DO
+ * 车型维保项目 DO
  *
  * @author 芋道源码
  */
@@ -35,12 +35,7 @@ public class VehicleModelMaintainItemDO extends BaseDO {
     private Long vehicleModelId;
 
     /**
-     * 维保项目ID
-     */
-    private Long maintainItemId;
-
-    /**
-     * 保养项目
+     * 保养项目名称
      */
     private String maintainItem;
 

yudao-module-asset/yudao-module-asset-server/src/main/java/cn/iocoder/yudao/module/asset/dal/mysql/vehiclemodel/VehicleModelMaintainItemMapper.java

@@ -3,6 +3,7 @@ package cn.iocoder.yudao.module.asset.dal.mysql.vehiclemodel;
 import cn.iocoder.yudao.framework.mybatis.core.mapper.BaseMapperX;
 import cn.iocoder.yudao.module.asset.dal.dataobject.vehiclemodel.VehicleModelMaintainItemDO;
 import org.apache.ibatis.annotations.Mapper;
+import org.apache.ibatis.annotations.Select;
 
 import java.util.List;
 
@@ -22,4 +23,10 @@ public interface VehicleModelMaintainItemMapper extends BaseMapperX<VehicleModel
         delete(VehicleModelMaintainItemDO::getVehicleModelId, vehicleModelId);
     }
 
+    /**
+     * 查询所有已使用的保养项目名称（去重）
+     */
+    @Select("SELECT DISTINCT maintain_item FROM asset_vehicle_model_maintain_item WHERE deleted = 0 AND maintain_item IS NOT NULL AND maintain_item != '' ORDER BY maintain_item")
+    List<String> selectDistinctMaintainItems();
+
 }

yudao-module-asset/yudao-module-asset-server/src/main/java/cn/iocoder/yudao/module/asset/service/vehiclemodel/VehicleModelService.java

@@ -85,4 +85,11 @@ public interface VehicleModelService {
      */
     List<VehicleModelMaintainItemDO> getVehicleModelMaintainItems(Long vehicleModelId);
 
+    /**
+     * 获取已使用的保养项目列表（去重）
+     *
+     * @return 保养项目名称列表
+     */
+    List<String> getUsedMaintainItems();
+
 }

yudao-module-asset/yudao-module-asset-server/src/main/java/cn/iocoder/yudao/module/asset/service/vehiclemodel/VehicleModelServiceImpl.java

@@ -114,6 +114,11 @@ public class VehicleModelServiceImpl implements VehicleModelService {
         return vehicleModelMaintainItemMapper.selectListByVehicleModelId(vehicleModelId);
     }
 
+    @Override
+    public List<String> getUsedMaintainItems() {
+        return vehicleModelMaintainItemMapper.selectDistinctMaintainItems();
+    }
+
     /**
      * 保存维保项目
      */