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/internal/handler/instance_group.go

@@ -46,13 +46,53 @@ func UpdateInstanceGroup(c *gin.Context) {
 		return
 	}
 
-	var input map[string]interface{}
+	var input struct {
+		Name             *string `json:"name"`
+		Description      *string `json:"description"`
+		DBType           *string `json:"db_type"`
+		Permissions      *string `json:"permissions"`
+		IsActive         *bool   `json:"is_active"`
+		WorkflowID       *int    `json:"workflow_id"`
+		AccessType       *string `json:"access_type"`
+		AllowedDatabases *string `json:"allowed_databases"`
+	}
 	if err := c.ShouldBindJSON(&input); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
 		return
 	}
 
-	if err := model.DB.Model(&group).Updates(input).Error; err != nil {
+	updates := map[string]interface{}{}
+	if input.Name != nil {
+		updates["name"] = *input.Name
+	}
+	if input.Description != nil {
+		updates["description"] = *input.Description
+	}
+	if input.DBType != nil {
+		updates["db_type"] = *input.DBType
+	}
+	if input.Permissions != nil {
+		updates["permissions"] = *input.Permissions
+	}
+	if input.IsActive != nil {
+		updates["is_active"] = *input.IsActive
+	}
+	if input.WorkflowID != nil {
+		updates["workflow_id"] = *input.WorkflowID
+	}
+	if input.AccessType != nil {
+		updates["access_type"] = *input.AccessType
+	}
+	if input.AllowedDatabases != nil {
+		updates["allowed_databases"] = *input.AllowedDatabases
+	}
+
+	if len(updates) == 0 {
+		c.JSON(http.StatusOK, enrichGroup(group))
+		return
+	}
+
+	if err := model.DB.Model(&group).Updates(updates).Error; err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
 		return
 	}