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

frontend/src/pages/WorkflowPage.tsx

@@ -625,7 +625,7 @@ export default function WorkflowPage() {
                             {wf.is_active ? '已启用' : '已禁用'}
                           </span>
                           <span className="px-2 py-0.5 bg-[var(--dg-bg-tertiary)] text-[var(--dg-text-muted)] text-xs rounded">
-                            {wf.steps.length} 个阶段
+                            {(wf.steps || []).length} 个阶段
                           </span>
                         </div>
 
@@ -633,9 +633,10 @@ export default function WorkflowPage() {
 
                         {/* Flow visualization */}
                         <div className="flex items-center gap-1 mt-4 p-4 bg-[var(--dg-bg-tertiary)] rounded-lg overflow-x-auto">
-                          {wf.steps.map((step, i) => {
+                          {(wf.steps || []).map((step, i) => {
                             const StepIcon = STEP_TYPE_ICONS[step.step_type] || Shield;
                             const uids: number[] = step.approver_ids ? JSON.parse(step.approver_ids) : [];
+                            const stepsArr = wf.steps || [];
                             return (
                               <div key={step.id || i} className="flex items-center gap-1 flex-shrink-0">
                                 <div className={`px-3 py-2 rounded-lg border ${STEP_TYPE_COLORS[step.step_type] || STEP_TYPE_COLORS.serial} flex items-center gap-2`}>
@@ -649,7 +650,7 @@ export default function WorkflowPage() {
                                     )}
                                   </div>
                                 </div>
-                                {i < wf.steps.length - 1 && <ArrowRight className="w-4 h-4 text-[var(--dg-text-muted)] flex-shrink-0" />}
+                                {i < stepsArr.length - 1 && <ArrowRight className="w-4 h-4 text-[var(--dg-text-muted)] flex-shrink-0" />}
                               </div>
                             );
                           })}
@@ -662,7 +663,7 @@ export default function WorkflowPage() {
                               <GitBranch className="w-4 h-4 text-[var(--dg-text-muted)]" />
                               流程步骤详情
                             </h4>
-                            {wf.steps.map((step, si) => {
+                            {(wf.steps || []).map((step, si) => {
                               const StepIcon = STEP_TYPE_ICONS[step.step_type] || Shield;
                               const colorClass = STEP_TYPE_COLORS[step.step_type] || STEP_TYPE_COLORS.serial;
                               const uids: number[] = step.approver_ids ? JSON.parse(step.approver_ids) : [];