fix: 实例分组编辑保存实例关联 — UpdateInstanceGroup 接收 readonly/write_instance_ids

- UpdateInstanceGroup 新增 readonly_instance_ids/write_instance_ids 字段处理
- 清除旧成员后重新写入 instance_group_members 表
- enrichGroup 返回 readonly_instance_ids/write_instance_ids 供前端穿梭框回显
- 新增 collectInstanceIDs helper 去重提取 ID 列表

frontend/src/pages/WorkflowPage.tsx

@@ -635,7 +635,8 @@ export default function WorkflowPage() {
                         <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) => {
                             const StepIcon = STEP_TYPE_ICONS[step.step_type] || Shield;
-                            const uids: number[] = step.approver_ids ? JSON.parse(step.approver_ids) : [];
+                            let uids: number[] = [];
+                            try { const parsed = JSON.parse(step.approver_ids); if (Array.isArray(parsed)) uids = parsed; } catch {}
                             const stepsArr = wf.steps || [];
                             return (
                               <div key={step.id || i} className="flex items-center gap-1 flex-shrink-0">