fix(backend): _recalc_product_location 修复 func.or_() → or_()

func 是 SQL 函数调用（如 func.now()），不能替代 WHERE 条件中的 or_ 运算符。
这会导致结束协助分支时 SQL 抛错 500。

backend/app/services/task_service.py

@@ -46,7 +46,7 @@ async def _recalc_product_location(
     优先级：
       WIP > PENDING > COMPLETED/ARCHIVED > None
     """
-    from sqlalchemy import select as sa_select, case as sa_case, func as sa_func
+    from sqlalchemy import select as sa_select, case as sa_case, or_ as sa_or_
 
     product_result = await db.execute(
         sa_select(Product).where(Product.id == product_id)
@@ -60,7 +60,7 @@ async def _recalc_product_location(
         sa_select(Task)
         .where(
             Task.product_id == product_id,
-            sa_func.or_(
+            sa_or_(
                 Task.parent_task_id.is_(None),
                 Task.task_type.in_(["TRANSFER", "RECOVERY"]),
             ),