feat(monitoring): add grafana, prometheus, node_exporter roles

- grafana: postgres backend, provisioning (prometheus datasource + node/NUT dashboards)
- prometheus: native service, scrape config via host_vars, localhost bind
- node_exporter: host metrics exporter (localhost:9100)

roles/node_exporter/handlers/main.yml

@@ -0,0 +1,5 @@
+---
+- name: Restart node_exporter
+  ansible.builtin.systemd:
+    name: "{{ node_exporter_service }}"
+    state: restarted