feat: add PDS backup service with rclone + S3

Three backup types: pre-update (Watchtower lifecycle hook),
pre-deploy (Ansible task), and periodic (systemd timer).
All uploads go to OVH S3 via rclone. Backs up SQLite DBs,
pds.env, and Caddy config.

👾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta Code <noreply@letta.com>

templates/backup.timer.j2

@@ -0,0 +1,10 @@
+[Unit]
+Description=Periodic PDS Backup
+
+[Timer]
+OnBootSec=5min
+OnUnitActiveSec={{ pds_backup_period }}
+Persistent=true
+
+[Install]
+WantedBy=timers.target