fix(stocktake): 修复盘点报告导出 500 错误

- 导出盘点报告时第1126行使用了 func.sum()/func.coalesce()
  但文件未导入 sqlalchemy 的 func
- 改为 db.func.sum()/db.func.coalesce()，与文件内其他用法一致

inventory-backend/app/api/v1/inbound/stock.py

@@ -1123,7 +1123,7 @@ def export_stocktake():
         borrow_rows = db.session.query(
             TransBorrow.source_table,
             TransBorrow.stock_id,
-            func.sum(func.coalesce(TransBorrow.quantity, 0) - func.coalesce(TransBorrow.returned_quantity, 0)).label('pending')
+            db.func.sum(db.func.coalesce(TransBorrow.quantity, 0) - db.func.coalesce(TransBorrow.returned_quantity, 0)).label('pending')
         ).filter(
             TransBorrow.is_returned == False
         ).group_by(