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/grafana/handlers/main.yml

@@ -0,0 +1,12 @@
+---
+# Grafana handlers
+
+- name: Restart grafana
+  ansible.builtin.systemd:
+    name: "{{ grafana_service }}"
+    state: restarted
+
+- name: Reload nginx
+  ansible.builtin.systemd:
+    name: nginx
+    state: reloaded