feat: 定时执行工单 — 新建工单+自动任务管理全面重构

后端:
- Ticket 模型新增 scheduled_at 字段 (DateTime, nullable)
- TicketStatus 新增 SCHEDULED 状态
- 状态转换图: approved→scheduled, scheduled→executing/approved
- execute_ticket: scheduled_at 在未来时进入 scheduled 状态
- ticket_scheduler.py: asyncio 后台定时执行器
- GET /tickets/scheduled/list: 列出定时执行中工单
- POST /tickets/{id}/cancel-schedule: 取消定时执行

前端:
- TicketCreatePage: 新增定时执行 checkbox + datetime-local picker (秒级精度)
- AutoTaskPage: 全面重写为真实定时任务管理页
  - 实时倒计时显示 (Xh Xm 后执行)
  - DB 类型图标 (MySQL/PG/Redis/MongoDB)
  - SQL 预览 + 取消定时执行
  - 每 30 秒自动刷新

frontend/src/pages/TicketCreatePage.tsx

@@ -98,6 +98,8 @@ export default function TicketCreatePage() {
   const [database, setDatabase] = useState('');
   const [sqlContent, setSqlContent] = useState('');
   const [reason, setReason] = useState('');
+  const [scheduledAt, setScheduledAt] = useState('');
+  const [enableSchedule, setEnableSchedule] = useState(false);
 
   // -- Data / Loading --
   const [instances, setInstances] = useState<InstanceItem[]>([]);
@@ -287,6 +289,7 @@ export default function TicketCreatePage() {
         sql_content: sqlContent,
         reason: reason.trim(),
         priority: 'medium',
+        scheduled_at: enableSchedule && scheduledAt ? new Date(scheduledAt).toISOString() : undefined,
       });
       navigate('/tickets');
     } catch (e: unknown) {
@@ -700,6 +703,27 @@ export default function TicketCreatePage() {
           </div>
         </div>
 
+        {/* 定时执行 */}
+        <div className="flex items-center gap-4">
+          <label className="flex items-center gap-2 cursor-pointer select-none">
+            <input type="checkbox" checked={enableSchedule} onChange={(e) => setEnableSchedule(e.target.checked)}
+              className="w-4 h-4 rounded border-[var(--dg-border)] bg-[var(--dg-bg-primary)] text-blue-500 focus:ring-blue-500 focus:ring-offset-0" />
+            <Clock className="w-4 h-4 text-[var(--dg-text-muted)]" />
+            <span className="text-sm">定时执行</span>
+          </label>
+          {enableSchedule && (
+            <input type="datetime-local" value={scheduledAt} onChange={(e) => setScheduledAt(e.target.value)}
+              step="1"
+              className="px-3 py-2 bg-[var(--dg-bg-primary)] border border-[var(--dg-border)] rounded-lg text-sm focus:outline-none focus:border-blue-500 transition-colors"
+              min={new Date().toISOString().slice(0, 16)} />
+          )}
+          {enableSchedule && scheduledAt && (
+            <span className="text-xs text-[var(--dg-text-muted)]">
+              将于 {new Date(scheduledAt).toLocaleString('zh-CN')} 执行
+            </span>
+          )}
+        </div>
+
         {/* SQL Editor + Browser Panel */}
         <div className="grid grid-cols-1 lg:grid-cols-5 gap-4">
           <div className="lg:col-span-3 space-y-3">