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/service/tenant.go

@@ -118,3 +118,46 @@ func HasAnyScope(scopes string, wanted ...string) bool {
 	}
 	return false
 }
+
+func NormalizeMerchantFeatures(features string) string {
+	set := ParseScopes(features)
+	if len(set) == 0 {
+		set = ParseScopes(model.DefaultMerchantFeatures)
+	}
+	valid := map[string]struct{}{
+		model.MerchantFeatureProducts:  {},
+		model.MerchantFeatureOrders:    {},
+		model.MerchantFeatureWallet:    {},
+		model.MerchantFeatureAPI:       {},
+		model.MerchantFeatureCallbacks: {},
+	}
+	ordered := []string{
+		model.MerchantFeatureProducts,
+		model.MerchantFeatureOrders,
+		model.MerchantFeatureWallet,
+		model.MerchantFeatureAPI,
+		model.MerchantFeatureCallbacks,
+	}
+	out := make([]string, 0, len(ordered))
+	for _, feature := range ordered {
+		if _, ok := set[feature]; ok {
+			if _, allowed := valid[feature]; allowed {
+				out = append(out, feature)
+			}
+		}
+	}
+	if len(out) == 0 {
+		return model.DefaultMerchantFeatures
+	}
+	return strings.Join(out, ",")
+}
+
+func MerchantHasFeature(features string, wanted ...string) bool {
+	set := ParseScopes(features)
+	for _, feature := range wanted {
+		if _, ok := set[feature]; ok {
+			return true
+		}
+	}
+	return false
+}