feat: 在制品加业务序列号 + 追溯码改名为身份证

1. 在制品行: 显示 external_serial（业务序列号）
   优先 external_serial → 兜底 product_sn
   旧: 装配 A1B2C3D4  张三
   新: 装配 25022  张三

2. 留言板: 追溯码 → 身份证
   旧: 追溯码: 000000000000000D
   新: 身份证: 000000000000000D

3. 后端: WipTask 新增 external_serial 字段
   关联查询 Product.external_serial

backend/app/services/dashboard_service.py

@@ -29,6 +29,7 @@ class WipTask(BaseModel):
     task_name: str
     assignee: str
     product_sn: str
+    external_serial: str | None   # 业务序列号
     status: str
     received_at: str
     duration_hours: float
@@ -131,7 +132,7 @@ async def get_wip_tasks(db: AsyncSession, limit: int = 20) -> list[WipTask]:
     from app.core.time_utils import get_beijing_time, BEIJING_TZ
 
     stmt = (
-        select(Task, Product.serial_number)
+        select(Task, Product.serial_number, Product.external_serial)
         .join(Product, Task.product_id == Product.id)
         .where(Task.status.in_([TASK_STATUS_PENDING, TASK_STATUS_WIP]))
         .limit(limit * 2)  # 多取一些，后面 Python 统一排序
@@ -147,7 +148,7 @@ async def get_wip_tasks(db: AsyncSession, limit: int = 20) -> list[WipTask]:
 
     now = get_beijing_time()
     wip_list: list[WipTask] = []
-    for task, product_sn in rows:
+    for task, product_sn, ext_sn in rows:
         start = task.received_at or task.created_at
         if start:
             if start.tzinfo is None:
@@ -165,6 +166,7 @@ async def get_wip_tasks(db: AsyncSession, limit: int = 20) -> list[WipTask]:
             task_name=task.task_name,
             assignee=name_map.get(task.assignee_id or "", task.assignee_id or "未分配"),
             product_sn=product_sn or "",
+            external_serial=ext_sn or None,
             status=task.status,
             received_at=recv_str,
             duration_hours=hours,