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/middleware/feature.go

@@ -0,0 +1,45 @@
+package middleware
+
+import (
+	"errors"
+
+	"affiliate_dash/internal/model"
+	"affiliate_dash/internal/pkg/response"
+	"affiliate_dash/internal/service"
+
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+// RequireMerchantFeature 校验当前商户是否开通指定功能。
+func RequireMerchantFeature(db *gorm.DB, features ...string) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		merchantID := GetMerchantID(c)
+		if merchantID == 0 {
+			response.Forbidden(c, "当前请求未绑定商户")
+			c.Abort()
+			return
+		}
+		var merchant model.Merchant
+		if err := db.Select("id", "status", "features").First(&merchant, merchantID).Error; err != nil {
+			if errors.Is(err, gorm.ErrRecordNotFound) {
+				response.Forbidden(c, "商户不存在")
+			} else {
+				response.ServerError(c, err.Error())
+			}
+			c.Abort()
+			return
+		}
+		if merchant.Status != model.MerchantStatusActive {
+			response.Forbidden(c, "商户已禁用")
+			c.Abort()
+			return
+		}
+		if !service.MerchantHasFeature(merchant.Features, features...) {
+			response.Forbidden(c, "商户功能未开通")
+			c.Abort()
+			return
+		}
+		c.Next()
+	}
+}