refactor(repo): 收敛 base_model 双轨，删除函数统一走 repo.batch_delete_r

- shared/base_repository.py 新增 batch_delete_r（返回 R，等价原 base_model 语义）
- 20 个模块删除函数从 Model.batch_delete_r 改为 x_repo.batch_delete_r
- shared/base_model.py 移除 classmethod（get_by_id/get_by_ids/get_all_active/exists/batch_delete_r），只留字段 + to_dict
- 清理 base_model 多余 imports
- 实测 dept 删除、批量删除及多模块读接口正常

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

modules/system/position/service.py

@@ -101,7 +101,7 @@ def update_position(request, data: PositionForm):
 # 删除岗位
 
 def delete_position(position_id):
-    return Position.batch_delete_r(position_id)
+    return position_repo.batch_delete_r(position_id)
 # 设置状态
 
 def update_position_status(request, data: PositionStatusForm):