feat(backup): 备份/恢复异步化，解决 504 超时

POST /backups 和 POST /backups/:id/restore 改为异步：立即返回 HTTP 202，
后台 goroutine 独立执行 pg_dump → gzip → S3 上传，前端每 2s 轮询状态。

后端:
- 新增 StartBackup/StartRestore 方法，后台 goroutine 不依赖 HTTP 连接
- Graceful shutdown 等待活跃操作完成，启动时清理孤立 running 记录
- BackupRecord 新增 progress/restore_status 字段支持进度和恢复状态追踪

前端:
- 创建备份/恢复后轮询 GET /backups/:id 直到完成或失败
- 标签页切换暂停/恢复轮询，组件卸载清理定时器
- 正确处理 409（备份进行中）和轮询超时

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/admin/backup.ts

@@ -29,6 +29,10 @@ export interface BackupRecord {
   started_at: string
   finished_at?: string
   expires_at?: string
+  progress?: string
+  restore_status?: string
+  restore_error?: string
+  restored_at?: string
 }
 
 export interface CreateBackupRequest {
@@ -69,7 +73,7 @@ export async function updateSchedule(config: BackupScheduleConfig): Promise<Back
 
 // Backup operations
 export async function createBackup(req?: CreateBackupRequest): Promise<BackupRecord> {
-  const { data } = await apiClient.post<BackupRecord>('/admin/backups', req || {}, { timeout: 600000 })
+  const { data } = await apiClient.post<BackupRecord>('/admin/backups', req || {})
   return data
 }
 
@@ -93,8 +97,9 @@ export async function getDownloadURL(id: string): Promise<{ url: string }> {
 }
 
 // Restore
-export async function restoreBackup(id: string, password: string): Promise<void> {
-  await apiClient.post(`/admin/backups/${id}/restore`, { password }, { timeout: 600000 })
+export async function restoreBackup(id: string, password: string): Promise<BackupRecord> {
+  const { data } = await apiClient.post<BackupRecord>(`/admin/backups/${id}/restore`, { password })
+  return data
 }
 
 export const backupAPI = {