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

frontend/src/pages/admin/AdminDashboard.tsx

@@ -89,7 +89,9 @@ function WipRow({ t }: { t: WipTask }) {
       <div className="min-w-0 flex-1">
         <div className="flex items-baseline gap-2">
           <span className="text-sm font-medium text-gray-700 truncate">{t.task_name}</span>
-          <span className="font-mono text-xs text-gray-400 shrink-0">{t.product_sn}</span>
+          <span className="text-xs text-gray-500 shrink-0">
+            {t.external_serial || t.product_sn}
+          </span>
           <span className="text-xs text-gray-400 shrink-0">{t.assignee}</span>
         </div>
         <div className="mt-0.5 flex items-center gap-2 text-[11px] text-gray-400">
@@ -135,7 +137,7 @@ function MsgRow({ m }: { m: ProductMessageItem }) {
                 <span className="font-medium text-gray-600">{m.external_serial || "未录入"}</span>
               </span>
             )}
-            <span className="text-[10px] text-gray-300 font-mono">追溯码: {m.product_sn}</span>
+            <span className="text-[10px] text-gray-300 font-mono">身份证: {m.product_sn}</span>
           </div>
         </div>
       </div>

frontend/src/services/dashboardApi.ts

@@ -20,6 +20,7 @@ export interface WipTask {
   task_name: string;
   assignee: string;
   product_sn: string;
+  external_serial: string | null;
   status: string;
   received_at: string;
   duration_hours: number;