add downloadable backup

scripts/backup.php

@@ -0,0 +1,22 @@
+<?php
+
+define('__ROOT__', dirname(dirname(__FILE__)));
+require_once(__ROOT__.'/scripts/common.php');
+
+$user = get_user();
+$home = get_home();
+ensure_authenticated('You must be authenticated to download backup files.');
+
+set_timezone();
+$date = new DateTime();
+$date_str = $date->format("Ymd\\THis");
+
+header("Content-Description: File Transfer");
+header("Content-Type: application/octet-stream");
+header("Content-Disposition: attachment; filename=\"backup-$date_str.tar\"");
+
+$err=null;
+set_time_limit(0);
+passthru("sudo -u $user $home/BirdNET-Pi/scripts/backup_data.sh -a backup -f -", $err);
+debug_log(strval($err));
+exit();