fix: 消息通知页不能点进详情 — 补全 product_serial_number

问题: 点击通知卡片跳转 detail?taskId=xxx，但 detail.vue onLoad 只认 serial 参数导致空白页

修复:
1. 后端 NotificationResponse 新增 product_serial_number 字段
2. 后端 notifications API 联表 tasks+products 批量填充 serial
3. 前端 notify/index.vue handleCardTap 优先用 product_serial_number 跳转，兜底从 content 正则解析
4. 前端 detail.vue onLoad 新增 taskId 兜底 → doQueryByTask 反查 product_sn

backend/app/schemas/notification.py

@@ -13,6 +13,7 @@ class NotificationResponse(BaseModel):
     content: str
     type: str
     task_id: uuid.UUID | None = None
+    product_serial_number: str | None = None
     is_read: bool
     created_at: datetime