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/merchant.go

@@ -330,25 +330,39 @@ func (h *MerchantHandler) ListPlatformMerchants(c *gin.Context) {
 }
 
 type createMerchantReq struct {
-	Code        string `json:"code" binding:"required"`
-	Name        string `json:"name" binding:"required"`
-	ContactName string `json:"contact_name"`
-	ContactInfo string `json:"contact_info"`
-	OwnerUserID uint   `json:"owner_user_id" binding:"required"`
+	Code           string `json:"code" binding:"required"`
+	Name           string `json:"name" binding:"required"`
+	ContactName    string `json:"contact_name"`
+	ContactInfo    string `json:"contact_info"`
+	OwnerUserID    uint   `json:"owner_user_id"`
+	OwnerUsername  string `json:"owner_username"`
+	OwnerPassword  string `json:"owner_password"`
+	OwnerNickname  string `json:"owner_nickname"`
+	Features       string `json:"features"`
+	FeeType        string `json:"fee_type"`
+	FeeRateBP      int64  `json:"fee_rate_bp"`
+	FeeFixedAmount int64  `json:"fee_fixed_amount"`
 }
 
 func (h *MerchantHandler) CreateMerchant(c *gin.Context) {
 	var req createMerchantReq
 	if err := c.ShouldBindJSON(&req); err != nil {
-		response.BadRequest(c, "参数错误：code、name、owner_user_id 必填")
+		response.BadRequest(c, "参数错误：code、name 必填")
 		return
 	}
 	merchant, err := h.merchantSvc.CreateMerchant(service.CreateMerchantInput{
-		Code:        req.Code,
-		Name:        req.Name,
-		ContactName: req.ContactName,
-		ContactInfo: req.ContactInfo,
-		OwnerUserID: req.OwnerUserID,
+		Code:           req.Code,
+		Name:           req.Name,
+		ContactName:    req.ContactName,
+		ContactInfo:    req.ContactInfo,
+		OwnerUserID:    req.OwnerUserID,
+		OwnerUsername:  req.OwnerUsername,
+		OwnerPassword:  req.OwnerPassword,
+		OwnerNickname:  req.OwnerNickname,
+		Features:       req.Features,
+		FeeType:        req.FeeType,
+		FeeRateBP:      req.FeeRateBP,
+		FeeFixedAmount: req.FeeFixedAmount,
 	}, middleware.GetUserID(c))
 	if err != nil {
 		response.BadRequest(c, err.Error())
@@ -357,6 +371,40 @@ func (h *MerchantHandler) CreateMerchant(c *gin.Context) {
 	response.OK(c, merchant)
 }
 
+type updateMerchantSettingsReq struct {
+	Name           *string `json:"name"`
+	Status         *string `json:"status"`
+	ContactName    *string `json:"contact_name"`
+	ContactInfo    *string `json:"contact_info"`
+	Features       *string `json:"features"`
+	FeeType        *string `json:"fee_type"`
+	FeeRateBP      *int64  `json:"fee_rate_bp"`
+	FeeFixedAmount *int64  `json:"fee_fixed_amount"`
+}
+
+func (h *MerchantHandler) UpdateMerchantSettings(c *gin.Context) {
+	id, _ := strconv.ParseUint(c.Param("id"), 10, 64)
+	var req updateMerchantSettingsReq
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "参数错误")
+		return
+	}
+	if err := h.merchantSvc.UpdateMerchantSettings(uint(id), service.UpdateMerchantSettingsInput{
+		Name:           req.Name,
+		Status:         req.Status,
+		ContactName:    req.ContactName,
+		ContactInfo:    req.ContactInfo,
+		Features:       req.Features,
+		FeeType:        req.FeeType,
+		FeeRateBP:      req.FeeRateBP,
+		FeeFixedAmount: req.FeeFixedAmount,
+	}, middleware.GetUserID(c)); err != nil {
+		response.BadRequest(c, err.Error())
+		return
+	}
+	response.OK(c, nil)
+}
+
 func (h *MerchantHandler) AddPlatformMerchantMember(c *gin.Context) {
 	var req addMemberReq
 	if err := c.ShouldBindJSON(&req); err != nil {