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/README.md

@@ -21,6 +21,19 @@ Keep localhost-only — Prometheus scrapes it locally on the same host. To add
 collectors (e.g. `--collector.systemd`, `--collector.textfile.directory`), extend
 `node_exporter_listen_address` or override the args in host_vars.
 
+## Collectors
+
+The role enables, on top of the defaults:
+
+- `--collector.zfs` — ZFS pool/dataset metrics.
+- `--collector.systemd` — unit state (`node_systemd_unit_state`), restart
+  counts (`node_systemd_restart_count_total`) and timer trigger times
+  (`node_systemd_timer_last_trigger_seconds` / `..._next_trigger_seconds`).
+  Powers the systemd dashboard (failed units, timers that stopped firing).
+- `--collector.textfile.directory={{ node_exporter_textfile_dir }}` — re-exposes
+  `*.prom` files dropped into that directory. The backup role writes
+  `backup_*` metrics there; other cron/timer jobs can do the same.
+
 ## Pairing with Prometheus
 
 ```yaml