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/i18n/locales/en.ts

@@ -1025,7 +1025,12 @@ export default {
         createBackup: 'Create Backup',
         backing: 'Backing up...',
         backupCreated: 'Backup created successfully',
-        expireDays: 'Expire Days'
+        expireDays: 'Expire Days',
+        alreadyInProgress: 'A backup is already in progress',
+        backupRunning: 'Backup in progress...',
+        backupFailed: 'Backup failed',
+        restoreRunning: 'Restore in progress...',
+        restoreFailed: 'Restore failed',
       },
       columns: {
         status: 'Status',
@@ -1042,6 +1047,11 @@ export default {
         completed: 'Completed',
         failed: 'Failed'
       },
+      progress: {
+        pending: 'Preparing',
+        dumping: 'Dumping database',
+        uploading: 'Uploading',
+      },
       trigger: {
         manual: 'Manual',
         scheduled: 'Scheduled'

frontend/src/i18n/locales/zh.ts

@@ -1047,7 +1047,12 @@ export default {
         createBackup: '创建备份',
         backing: '备份中...',
         backupCreated: '备份创建成功',
-        expireDays: '过期天数'
+        expireDays: '过期天数',
+        alreadyInProgress: '已有备份正在进行中',
+        backupRunning: '备份进行中...',
+        backupFailed: '备份失败',
+        restoreRunning: '恢复进行中...',
+        restoreFailed: '恢复失败',
       },
       columns: {
         status: '状态',
@@ -1064,6 +1069,11 @@ export default {
         completed: '已完成',
         failed: '失败'
       },
+      progress: {
+        pending: '准备中',
+        dumping: '导出数据库',
+        uploading: '上传中',
+      },
       trigger: {
         manual: '手动',
         scheduled: '定时'