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.service.j2

@@ -0,0 +1,8 @@
+[Unit]
+Description=PDS Backup Service
+After=docker.service
+Requires=docker.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/bin/backup.sh periodic