fix: 修复3个前端报错 — 实例分组编辑500/定时任务404/工作流null

1. instance_group.go: UpdateInstanceGroup 改用结构体输入,过滤系统字段,防止GORM更新主键导致500
2. scheduled_task.go: 新增 ListScheduledTasks + CancelScheduledTask handler
3. router.go: 注册 /tickets/scheduled/list 和 /tickets/:id/cancel-schedule 路由
4. WorkflowPage.tsx: 4处 wf.steps 添加 || [] 空值保护

backend/router/router.go

@@ -27,10 +27,12 @@ func Setup(r *gin.Engine) {
 		// Tickets
 		auth.GET("/tickets/statistics", handler.TicketStatistics)
 		auth.GET("/tickets", handler.ListTickets)
+		auth.GET("/tickets/scheduled/list", handler.ListScheduledTasks)
 		auth.GET("/tickets/:id", handler.GetTicket)
 		auth.POST("/tickets", handler.CreateTicket)
 		auth.POST("/tickets/:id/approve", handler.ApproveTicket)
 		auth.POST("/tickets/:id/execute", handler.ExecuteTicket)
+		auth.POST("/tickets/:id/cancel-schedule", handler.CancelScheduledTask)
 
 		// Instances
 		auth.GET("/instances", handler.ListInstances)