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/auth.go

@@ -2,9 +2,6 @@ package service
 
 import (
 	"errors"
-	"fmt"
-	"math/rand"
-	"time"
 
 	"affiliate_dash/internal/model"
 	"affiliate_dash/internal/pkg/jwt"
@@ -63,9 +60,8 @@ func (s *AuthService) Register(username, password, nickname string) (*model.User
 		Username:     username,
 		PasswordHash: string(hash),
 		Nickname:     nickname,
-		Role:         model.RoleDistributor,
+		Role:         model.RoleMerchant,
 		Status:       1,
-		InviteCode:   generateInviteCode(),
 	}
 	if user.Nickname == "" {
 		user.Nickname = username
@@ -116,7 +112,6 @@ func (s *AuthService) EnsureAdmin() error {
 		Nickname:     "管理员",
 		Role:         model.RoleAdmin,
 		Status:       1,
-		InviteCode:   "ADMIN001",
 	}
 	if err := s.db.Create(admin).Error; err != nil {
 		return err
@@ -126,8 +121,3 @@ func (s *AuthService) EnsureAdmin() error {
 	}
 	return nil
 }
-
-func generateInviteCode() string {
-	r := rand.New(rand.NewSource(time.Now().UnixNano()))
-	return fmt.Sprintf("D%06d", r.Intn(1000000))
-}