add downloadable backup

scripts/system_controls.php

@@ -49,7 +49,7 @@ function update() {
     <button type="submit" name="submit" value="sudo clear_all_data.sh" onclick="return confirm('Clear ALL Data? Note that this cannot be undone and will take up to 90 seconds.')">Clear ALL data</button>
   </div>
 </form>
-</div>
+<div><a href="scripts/backup.php" download ><button onclick="return confirm('Download backup? Note that this could take a long time.')">Download backup</button></a></div>
 <?php
   $cmd="cd ".$home."/BirdNET-Pi && sudo -u ".$user." git rev-list --max-count=1 HEAD";
   $curr_hash = shell_exec($cmd);