重构: 手续费改为百分比/固定二选一，清理旧 Skin/Order/ShipLog 演示链路

手续费:
- 新增 fee_type 字段(rate/fixed)，百分比与固定金额二选一，不再叠加
- calculateServiceFee 按 fee_type 分支计算
- 商户创建/更新校验 fee_type，订单落库快照 fee_type
- 前端表单改为下拉选择手续费类型，动态显示对应输入框
- 测试拆为 TestFeeRate + TestFeeFixed

清理旧链路:
- 删除旧 Skin/Order/ShipLog 模型及 OrderService/SkinService
- Dashboard 迁移到 FulfillmentService
- 上游 /api/open/v1 改为基于 FulfillmentOrder 实现，接口契约不变
- 推送留痕改用 AuditLog，不再建 ShipLog 表
- 删除前端 Skins/Orders/ShipLogs/Distributors 页面及路由、菜单、API、类型
- 新增迁移 003: 添加 fee_type 列并 DROP 旧表

frontend/src/pages/Dashboard.tsx

@@ -1,8 +1,7 @@
 import { useEffect, useState } from 'react'
 import { Card, Col, Row, Statistic, Typography, Spin, message } from 'antd'
 import {
-  SkinOutlined,
-  TeamOutlined,
+  ShopOutlined,
   ShoppingOutlined,
   DollarOutlined,
   PercentageOutlined,
@@ -39,12 +38,12 @@ export default function Dashboard() {
       <Row gutter={[16, 16]}>
         <Col xs={24} sm={12} lg={8}>
           <Card>
-            <Statistic title="皮肤商品" value={stats?.skin_count ?? 0} prefix={<SkinOutlined />} />
+            <Statistic title="商户商品" value={stats?.product_count ?? 0} prefix={<ShoppingOutlined />} />
           </Card>
         </Col>
         <Col xs={24} sm={12} lg={8}>
           <Card>
-            <Statistic title="分销商" value={stats?.distributor_count ?? 0} prefix={<TeamOutlined />} />
+            <Statistic title="平台商户" value={stats?.merchant_count ?? 0} prefix={<ShopOutlined />} />
           </Card>
         </Col>
         <Col xs={24} sm={12} lg={8}>
@@ -66,8 +65,8 @@ export default function Dashboard() {
         <Col xs={24} sm={12} lg={8}>
           <Card>
             <Statistic
-              title="累计佣金"
-              value={stats?.total_commission ?? 0}
+              title="平台手续费"
+              value={stats?.total_fees ?? 0}
               precision={2}
               prefix={<PercentageOutlined />}
               suffix="元"