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

backend/app/schemas/ticket.py

@@ -12,6 +12,7 @@ class TicketCreate(BaseModel):
     rollback_sql: Optional[str] = None
     reason: str = Field(..., min_length=5)
     priority: Literal["high", "medium", "low"] = "medium"
+    scheduled_at: Optional[datetime] = None
 
 
 class TicketReview(BaseModel):
@@ -42,6 +43,7 @@ class TicketResponse(BaseModel):
     create_time: datetime
     review_time: Optional[datetime] = None
     execute_time: Optional[datetime] = None
+    scheduled_at: Optional[datetime] = None
 
     model_config = ConfigDict(from_attributes=True)