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/cmd/server/main.go

@@ -50,8 +50,6 @@ func main() {
 	}
 	tenantSvc := service.NewTenantService(db)
 	authSvc := service.NewAuthService(db, jm, tenantSvc)
-	skinSvc := service.NewSkinService(db)
-	orderSvc := service.NewOrderService(db)
 	userSvc := service.NewUserService(db, tenantSvc)
 	callbackSvc := service.NewCallbackService(db, codec)
 	fulfillmentSvc := service.NewFulfillmentService(db, callbackSvc)
@@ -60,19 +58,15 @@ func main() {
 	if err := authSvc.EnsureAdmin(); err != nil {
 		log.Fatalf("ensure admin: %v", err)
 	}
-	if err := skinSvc.SeedCatalog(); err != nil {
-		log.Printf("seed skins: %v", err)
-	}
 
 	openlog.Init(cfg.OpenAPIDebug)
 
 	h := &router.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:           jm,
 		Tenant:        tenantSvc,
@@ -88,7 +82,7 @@ func main() {
 
 	r := router.Setup(h)
 	addr := ":" + cfg.Port
-	log.Printf("游戏皮肤分销系统 API 启动: http://localhost%s", addr)
+	log.Printf("游戏皮肤供货平台 API 启动: http://localhost%s", addr)
 	log.Printf("数据库: PostgreSQL")
 	log.Printf("默认管理员: admin / admin123")
 	log.Printf("开放接口鉴权: X-App-Key + X-Timestamp + X-Nonce + X-Sign (HMAC-SHA256)")