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/fulfillment_test.go

@@ -117,6 +117,80 @@ func TestFulfillmentCreateOrderDebitsWalletAndIsIdempotent(t *testing.T) {
 	}
 }
 
+func TestFulfillmentCreateOrderAppliesMerchantFeeRate(t *testing.T) {
+	db := newServiceTestDB(t)
+	merchantID, product := seedFulfillmentMerchant(t, db, "merchant-fee-rate", 1000, 5, 200)
+	if err := db.Model(&model.Merchant{}).Where("id = ?", merchantID).Updates(map[string]interface{}{
+		"fee_type":   model.FeeTypeRate,
+		"fee_rate_bp": int64(250),
+	}).Error; err != nil {
+		t.Fatalf("update merchant fee: %v", err)
+	}
+	svc := NewFulfillmentService(db, nil)
+
+	created, err := svc.CreateOrder(CreateFulfillmentOrderInput{
+		MerchantID:    merchantID,
+		APIClientID:   21,
+		ClientOrderNo: "client-fee-rate",
+		SKU:           product.SKU,
+		Quantity:      2,
+	})
+	if err != nil {
+		t.Fatalf("create order: %v", err)
+	}
+	// baseAmount = 200 * 2 = 400; rate 250BP = 400 * 250 / 10000 = 10; total = 410
+	if created.Order.BaseAmount != 400 || created.Order.ServiceFeeAmount != 10 || created.Order.Amount != 410 {
+		t.Fatalf("unexpected rate fee snapshot: %+v", created.Order)
+	}
+	if created.Order.FeeType != model.FeeTypeRate || created.Order.FeeRateBP != 250 {
+		t.Fatalf("unexpected rate fee config snapshot: %+v", created.Order)
+	}
+	var wallet model.WalletAccount
+	if err := db.Where("merchant_id = ?", merchantID).First(&wallet).Error; err != nil {
+		t.Fatalf("query wallet: %v", err)
+	}
+	if wallet.AvailableBalance != 590 {
+		t.Fatalf("wallet should debit total amount, got %d", wallet.AvailableBalance)
+	}
+}
+
+func TestFulfillmentCreateOrderAppliesMerchantFeeFixed(t *testing.T) {
+	db := newServiceTestDB(t)
+	merchantID, product := seedFulfillmentMerchant(t, db, "merchant-fee-fixed", 1000, 5, 200)
+	if err := db.Model(&model.Merchant{}).Where("id = ?", merchantID).Updates(map[string]interface{}{
+		"fee_type":         model.FeeTypeFixed,
+		"fee_fixed_amount": int64(30),
+	}).Error; err != nil {
+		t.Fatalf("update merchant fee: %v", err)
+	}
+	svc := NewFulfillmentService(db, nil)
+
+	created, err := svc.CreateOrder(CreateFulfillmentOrderInput{
+		MerchantID:    merchantID,
+		APIClientID:   22,
+		ClientOrderNo: "client-fee-fixed",
+		SKU:           product.SKU,
+		Quantity:      2,
+	})
+	if err != nil {
+		t.Fatalf("create order: %v", err)
+	}
+	// baseAmount = 400; fixed fee = 30; total = 430
+	if created.Order.BaseAmount != 400 || created.Order.ServiceFeeAmount != 30 || created.Order.Amount != 430 {
+		t.Fatalf("unexpected fixed fee snapshot: %+v", created.Order)
+	}
+	if created.Order.FeeType != model.FeeTypeFixed || created.Order.FeeFixedAmount != 30 {
+		t.Fatalf("unexpected fixed fee config snapshot: %+v", created.Order)
+	}
+	var wallet model.WalletAccount
+	if err := db.Where("merchant_id = ?", merchantID).First(&wallet).Error; err != nil {
+		t.Fatalf("query wallet: %v", err)
+	}
+	if wallet.AvailableBalance != 570 {
+		t.Fatalf("wallet should debit total amount, got %d", wallet.AvailableBalance)
+	}
+}
+
 func TestFulfillmentCancelRefundsOnceAndRestoresStock(t *testing.T) {
 	db := newServiceTestDB(t)
 	merchantID, product := seedFulfillmentMerchant(t, db, "merchant-b", 1000, 2, 300)