hotfix: get_product_by_serial补全func导入 — 修复500错误

backend/app/services/product_service.py

@@ -102,6 +102,7 @@ async def get_product_by_serial(db: AsyncSession, serial_number: str) -> Product
     creator_map: dict[uuid.UUID, str] = {}
     if all_task_ids:
         from app.models.task_log import TaskLog
+        from sqlalchemy import func
         # 每个 task 取最早的 create 日志的 operator_id
         sub = (
             select(