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/handler/open.go

@@ -11,13 +11,13 @@ import (
 	"github.com/gin-gonic/gin"
 )
 
-// OpenHandler 皮肤源头开放接口
+// OpenHandler 皮肤源头开放接口（上游 SourceOpen），基于 FulfillmentOrder 履约模型。
 type OpenHandler struct {
-	orderSvc *service.OrderService
+	fulfillmentSvc *service.FulfillmentService
 }
 
-func NewOpenHandler(orderSvc *service.OrderService) *OpenHandler {
-	return &OpenHandler{orderSvc: orderSvc}
+func NewOpenHandler(fulfillmentSvc *service.FulfillmentService) *OpenHandler {
+	return &OpenHandler{fulfillmentSvc: fulfillmentSvc}
 }
 
 // QueryOrder GET /api/open/v1/orders/:order_no
@@ -29,7 +29,7 @@ func (h *OpenHandler) QueryOrder(c *gin.Context) {
 	}
 	openlog.Info(c, "query_order start order_no=%s", orderNo)
 
-	data, err := h.orderSvc.QueryOpenOrder(orderNo)
+	data, err := h.fulfillmentSvc.QueryOpenOrder(orderNo)
 	if err != nil {
 		openlog.Warn(c, "query_order fail order_no=%s err=%s", orderNo, err.Error())
 		if err.Error() == "订单不存在" {
@@ -90,7 +90,7 @@ func (h *OpenHandler) ShipNotify(c *gin.Context) {
 		shippedAt = &t
 	}
 
-	result, err := h.orderSvc.HandleShipNotify(service.ShipNotifyInput{
+	result, err := h.fulfillmentSvc.HandleShipNotify(service.ShipNotifyInput{
 		OrderNo:         req.OrderNo,
 		ShipStatus:      req.ShipStatus,
 		ProviderOrderNo: req.ProviderOrderNo,