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>

backend/internal/handler/admin/backup_handler.go

@@ -98,12 +98,12 @@ func (h *BackupHandler) CreateBackup(c *gin.Context) {
 		expireDays = *req.ExpireDays
 	}
 
-	record, err := h.backupService.CreateBackup(c.Request.Context(), "manual", expireDays)
+	record, err := h.backupService.StartBackup(c.Request.Context(), "manual", expireDays)
 	if err != nil {
 		response.ErrorFrom(c, err)
 		return
 	}
-	response.Success(c, record)
+	response.Accepted(c, record)
 }
 
 func (h *BackupHandler) ListBackups(c *gin.Context) {
@@ -196,9 +196,10 @@ func (h *BackupHandler) RestoreBackup(c *gin.Context) {
 		return
 	}
 
-	if err := h.backupService.RestoreBackup(c.Request.Context(), backupID); err != nil {
+	record, err := h.backupService.StartRestore(c.Request.Context(), backupID)
+	if err != nil {
 		response.ErrorFrom(c, err)
 		return
 	}
-	response.Success(c, gin.H{"restored": true})
+	response.Accepted(c, record)
 }