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>

vars/main.yml.example

@@ -19,6 +19,14 @@ pds_invite_required: true
 # pds_blobstore_s3_access_key: CHANGE_ME
 # pds_blobstore_s3_secret_key: CHANGE_ME
 
+# Backup (defaults shown)
+# pds_backup_period: 24h
+# pds_backup_s3_bucket: my-pds-backup
+# pds_backup_s3_region: bhs
+# pds_backup_s3_endpoint: https://s3.bhs.io.cloud.ovh.net
+# pds_backup_s3_access_key: CHANGE_ME
+# pds_backup_s3_secret_key: CHANGE_ME
+
 # Email (uncomment to enable email verification)
 # pds_email_smtp_url: smtps://username:password@smtp.example.com/
 # pds_email_from_address: admin@example.com