修复: rclone 后端列表不显示 + 调度审计 + 批量删除

1. 修复前端 rclone 后端 API 路径双重 /api 前缀导致 404，
   存储类型下拉框现在正确显示全部 70+ rclone 后端
2. 调度器自动触发的备份任务计入审计日志（用户名: system）
3. 新增备份记录批量删除 API (POST /api/backup/records/batch-delete)

server/internal/http/router.go

@@ -106,6 +106,7 @@ func NewRouter(deps RouterDependencies) *gin.Engine {
 		backupRecords.GET("/:id/logs/stream", backupRecordHandler.StreamLogs)
 		backupRecords.GET("/:id/download", backupRecordHandler.Download)
 		backupRecords.POST("/:id/restore", backupRecordHandler.Restore)
+		backupRecords.POST("/batch-delete", backupRecordHandler.BatchDelete)
 		backupRecords.DELETE("/:id", backupRecordHandler.Delete)
 		dashboard := api.Group("/dashboard")
 		dashboard.Use(AuthMiddleware(deps.JWTManager))