feat: added sales_items and external_products into shipments list api

docs/2026-01-15_summary.md

@@ -40,5 +40,6 @@
 - 补齐出货单查询接口：
   - `GET /api/v1/shipment/shipments/`：出货单列表（merchant 隔离 + LimitOffsetPagination）
   - `GET /api/v1/shipment/shipments/<id>/`：出货单详情（merchant 隔离）
+- 出货单列表序列化器增强：`sales_items` 与 `external_finished_products` 字段稳定输出（无数据也返回空数组）
 - 更新 `docs/shipment_api.md`：补充“查询出货单（列表/详情）”说明
 - 新增 API 测试：覆盖列表/详情的商户隔离与未登录行为

docs/shipment_api.md

@@ -49,6 +49,8 @@
       "shipment_date": "2026-01-14",
       "remark": "备注信息",
       "items_count": 3,
+      "sales_items": [],
+      "external_finished_products": [],
       "created_by_id": 1,
       "created_by_name": "张三",
       "created_at": "2026-01-14T10:00:00Z",
@@ -108,6 +110,8 @@
     "shipment_date": "2026-01-14",
     "remark": "备注信息",
     "items_count": 3,
+    "sales_items": [],
+    "external_finished_products": [],
     "created_by_id": 1,
     "created_by_name": "张三",
     "created_at": "2026-01-14T10:00:00Z",