回调优化为单地址 upsert 配置并支持重置密钥

- 回调订阅改为单记录 upsert：保存即覆盖（URL/事件/状态），自动停用其他订阅，无新增/删除
- 新增重置密钥：保存时传 rotate_secret=true 重新生成密钥并返回一次，解决密钥丢失无法找回
- 删除不再使用的 PATCH /callbacks/:id/status 路由及对应 handler/service 死代码
- 前端回调页改为内联表单（URL/事件/状态），新增重置密钥按钮（确认后展示新密钥一次）
- 补充单地址复用、disabled 不投递、密钥轮换测试

backend/internal/service/fulfillment_test.go

@@ -559,7 +559,7 @@ func TestMarkProcessingTimeoutDistinguishesUpstreamSubmission(t *testing.T) {
 		t.Fatalf("create submitted order: %v", err)
 	}
 	if _, err := svc.UpdateFulfillment(FulfillmentUpdateInput{
-		MerchantID: merchantID,
+		MerchantID:  merchantID,
 		APIClientID: 13,
 		OrderNo:     submittedOrder.Order.OrderNo,
 		Status:      model.OrderStatusDelivering,
@@ -582,7 +582,7 @@ func TestMarkProcessingTimeoutDistinguishesUpstreamSubmission(t *testing.T) {
 		t.Fatalf("create interrupted order: %v", err)
 	}
 	if _, err := svc.UpdateFulfillment(FulfillmentUpdateInput{
-		MerchantID: merchantID,
+		MerchantID:  merchantID,
 		APIClientID: 13,
 		OrderNo:     interruptedOrder.Order.OrderNo,
 		Status:      model.OrderStatusDelivering,
@@ -805,6 +805,31 @@ func TestCreateTestOrderSupportsFailedFulfillableStatus(t *testing.T) {
 	}
 }
 
+func TestCreateTestOrderSupportsCancelledUnfulfillableStatus(t *testing.T) {
+	db := newServiceTestDB(t)
+	merchantID, product := seedFulfillmentMerchant(t, db, "merchant-test-cancelled", 0, -1, 100)
+	svc := NewFulfillmentService(db, nil)
+
+	order, err := svc.CreateTestOrder(CreateTestOrderInput{
+		MerchantID:  merchantID,
+		SKU:         product.SKU,
+		OrderStatus: model.OrderStatusCancelled,
+	})
+	if err != nil {
+		t.Fatalf("create cancelled test order: %v", err)
+	}
+	if order.OrderStatus != model.OrderStatusCancelled || order.CancelledAt == nil {
+		t.Fatalf("unexpected cancelled test order: %+v", order)
+	}
+	openOrder, err := svc.QueryOpenOrder(order.OrderNo)
+	if err != nil {
+		t.Fatalf("query open cancelled order: %v", err)
+	}
+	if openOrder.CanShip || openOrder.Status != "cancelled" || !strings.Contains(openOrder.CannotShipReason, "已取消") {
+		t.Fatalf("cancelled test order should be unfulfillable, got %+v", openOrder)
+	}
+}
+
 func TestHandleShipNotifyUpdatesOrderAndEnqueuesMerchantCallback(t *testing.T) {
 	db := newServiceTestDB(t)
 	merchantID, product := seedFulfillmentMerchant(t, db, "merchant-source-notify", 1000, 2, 100)