feat(monitoring): add backup and systemd metrics, alerts, and dashboards

- backup role emits last-success/run metrics into node_exporter's textfile collector
- node_exporter enables systemd and textfile collectors
- prometheus deploys BackupStale/BackupTimerSilent/SystemdUnitFailed alert rules
- grafana gains Backups - Restic and Systemd dashboards
- fix prometheus.yml params rendering (quote values so {job=...} stays a string)

roles/node_exporter/defaults/main.yml

@@ -4,3 +4,8 @@
 # Address the exporter listens on. Keep localhost-only — Prometheus scrapes it
 # locally on the same host; no need to expose on the network.
 node_exporter_listen_address: "127.0.0.1:9100"
+
+# Directory the textfile collector reads `*.prom` files from. Jobs (e.g. the
+# backup role) drop metrics here; node_exporter re-exposes them to Prometheus.
+# Must match backup_metrics_dir in roles/backup/defaults/main.yml.
+node_exporter_textfile_dir: /var/lib/node_exporter/textfile_collector