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/grafana/tasks/main.yml

@@ -248,6 +248,18 @@
     - wireguard-dashboard.json
   notify: Restart grafana
 
+- name: Deploy backup and systemd dashboards
+  ansible.builtin.copy:
+    src: "{{ item }}"
+    dest: "{{ grafana_provisioning_path }}/dashboards/{{ item }}"
+    owner: "{{ grafana_user }}"
+    group: "{{ grafana_group }}"
+    mode: "0644"
+  loop:
+    - backup-dashboard.json
+    - systemd-dashboard.json
+  notify: Restart grafana
+
 # Service management
 - name: Enable and start Grafana service
   ansible.builtin.systemd: