fix: 留言板区分业务序列号与系统追溯码

语义修正:
  Product.serial_number  = 16位HEX系统追溯码(如000000000000000D)
  Product.external_serial = 业务产品序列号(如25022)

后端:
  - ProductMessageItem 新增 external_serial 字段
  - search_product_messages 关联查询 Product.external_serial
  - 关键词搜索也支持按 external_serial 匹配

前端显示:
  样品升降台V1J · 序列号: 25022          ← 醒目
  追溯码: 000000000000000D               ← 10px浅灰小字

frontend/src/pages/admin/AdminDashboard.tsx

@@ -127,11 +127,15 @@ function MsgRow({ m }: { m: ProductMessageItem }) {
             <span className="shrink-0 text-xs text-gray-400">{t}</span>
           </div>
           <p className="mb-2 text-sm leading-relaxed text-gray-600">{m.content}</p>
-          <div className="flex items-center gap-1 text-xs text-gray-500">
+          <div className="flex flex-col gap-0.5">
             {m.material_name && (
-              <span className="font-medium text-gray-700">{m.material_name}</span>
+              <span className="text-xs">
+                <span className="font-semibold text-gray-700">{m.material_name}</span>
+                <span className="text-gray-400"> · 序列号: </span>
+                <span className="font-medium text-gray-600">{m.external_serial || "未录入"}</span>
+              </span>
             )}
-            <span className="font-mono text-gray-400">(SN: {m.product_sn})</span>
+            <span className="text-[10px] text-gray-300 font-mono">追溯码: {m.product_sn}</span>
           </div>
         </div>
       </div>

frontend/src/services/dashboardApi.ts

@@ -30,6 +30,7 @@ export interface ProductMessageItem {
   content: string;
   operator_name: string;
   product_sn: string;
+  external_serial: string | null;
   material_name: string;
   created_at: string;
 }