Manage own borgmatic systemd service and timer

Ship borgmatic.service and borgmatic.timer from the backup role instead
of relying on the package-provided units. The units are deployed to
/etc/systemd/system (overriding the package units), with a configurable
schedule via role defaults.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

roles/backup/templates/borgmatic.timer.j2

@@ -0,0 +1,11 @@
+# Managed by Ansible — do not edit by hand.
+[Unit]
+Description=Run borgmatic backup
+
+[Timer]
+OnCalendar={{ borgmatic_oncalendar }}
+RandomizedDelaySec={{ borgmatic_randomized_delay_sec }}
+Persistent={{ borgmatic_persistent | bool | lower }}
+
+[Install]
+WantedBy=timers.target