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/user.go

@@ -58,14 +58,17 @@ func (s *UserService) List(q UserListQuery) ([]model.User, int64, error) {
 	return list, total, err
 }
 
-func (s *UserService) Create(username, password, nickname, role string, parentID *uint, merchantID uint) (*model.User, error) {
+func (s *UserService) Create(username, password, nickname, role string, merchantID uint) (*model.User, error) {
 	var count int64
 	s.db.Model(&model.User{}).Where("username = ?", username).Count(&count)
 	if count > 0 {
 		return nil, errors.New("用户名已存在")
 	}
 	if role == "" {
-		role = model.RoleDistributor
+		role = model.RoleMerchant
+	}
+	if role != model.RoleAdmin && role != model.RoleMerchant {
+		return nil, errors.New("无效的账号角色")
 	}
 	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
 	if err != nil {
@@ -77,8 +80,6 @@ func (s *UserService) Create(username, password, nickname, role string, parentID
 		Nickname:     nickname,
 		Role:         role,
 		Status:       1,
-		InviteCode:   generateInviteCode(),
-		ParentID:     parentID,
 	}
 	if user.Nickname == "" {
 		user.Nickname = username