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

backend/internal/router/router_test.go

@@ -20,8 +20,6 @@ func TestSetupDoesNotPanic(t *testing.T) {
 	}
 	tenantSvc := service.NewTenantService(db)
 	authSvc := service.NewAuthService(db, jwt.NewManager("test-jwt"), tenantSvc)
-	skinSvc := service.NewSkinService(db)
-	orderSvc := service.NewOrderService(db)
 	userSvc := service.NewUserService(db, tenantSvc)
 	callbackSvc := service.NewCallbackService(db, codec)
 	fulfillmentSvc := service.NewFulfillmentService(db, callbackSvc)
@@ -34,11 +32,10 @@ func TestSetupDoesNotPanic(t *testing.T) {
 	}()
 	_ = Setup(&Handlers{
 		Auth:          handler.NewAuthHandler(authSvc),
-		Skin:          handler.NewSkinHandler(skinSvc),
-		Order:         handler.NewOrderHandler(orderSvc),
+		Dashboard:     handler.NewDashboardHandler(fulfillmentSvc),
 		User:          handler.NewUserHandler(userSvc),
 		Open:          handler.NewOpenV1Handler(merchantSvc, fulfillmentSvc),
-		SourceOpen:    handler.NewOpenHandler(orderSvc),
+		SourceOpen:    handler.NewOpenHandler(fulfillmentSvc),
 		Merchant:      handler.NewMerchantHandler(merchantSvc, fulfillmentSvc, callbackSvc),
 		JWT:           jwt.NewManager("test-jwt"),
 		Tenant:        tenantSvc,