Add restic backup stack with daily systemd timer

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_backup/stacks-backup.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Backup stacks data to HiDrive via restic
+After=network-online.target docker.service
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+User=root
+ExecStart=/root/stacks/_backup/backup.sh
+Environment=HOME=/root