fix(sgx): treat rsync exit code 24 as success in backup

Files vanishing during transfer is expected for mail directories
where messages are constantly moved.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

systems/x86_64-linux/sgx/backup.nix

@@ -29,6 +29,10 @@ in
     ];
     backupPrepareCommand = ''
       HOME=/root ${pkgs.rsync}/bin/rsync -e "${pkgs.openssh}/bin/ssh"  --exclude-from /root/excludelist --no-specials --no-devices --numeric-ids --delete-after --partial -axz backup@mx.surfsite.org:/{etc,var,home,root} ${backup_new_path}
+      rc=$?
+      # exit code 24 = files vanished during transfer (normal for mail dirs)
+      if [ $rc -eq 24 ]; then exit 0; fi
+      exit $rc
     '';
   };