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/App.tsx

@@ -6,15 +6,10 @@ import MainLayout from './layouts/MainLayout'
 import Login from './pages/Login'
 import Register from './pages/Register'
 import Dashboard from './pages/Dashboard'
-import Skins from './pages/Skins'
-import Orders from './pages/Orders'
-import Distributors from './pages/Distributors'
-import ShipLogs from './pages/ShipLogs'
 import OpenApiDocs from './pages/OpenApiDocs'
 import MerchantCenter from './pages/MerchantCenter'
 import PlatformMerchants from './pages/PlatformMerchants'
 import type { ReactNode } from 'react'
-
 function PrivateRoute({ children }: { children: ReactNode }) {
   const { token } = useAuth()
   if (!token) return <Navigate to="/login" replace />
@@ -41,17 +36,7 @@ function AppRoutes() {
         }
       >
         <Route index element={<Dashboard />} />
-        <Route path="skins" element={<Skins />} />
-        <Route path="orders" element={<Orders />} />
         <Route path="merchant-center" element={<MerchantCenter />} />
-        <Route
-          path="distributors"
-          element={
-            <AdminRoute>
-              <Distributors />
-            </AdminRoute>
-          }
-        />
         <Route
           path="platform-merchants"
           element={
@@ -60,14 +45,6 @@ function AppRoutes() {
             </AdminRoute>
           }
         />
-        <Route
-          path="ship-logs"
-          element={
-            <AdminRoute>
-              <ShipLogs />
-            </AdminRoute>
-          }
-        />
         <Route
           path="open-api"
           element={