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/prometheus/handlers/main.yml

@@ -0,0 +1,10 @@
+---
+- name: Restart Prometheus
+  ansible.builtin.systemd:
+      name: "{{ prometheus_service }}"
+      state: restarted
+
+- name: Reload Nginx
+  ansible.builtin.systemd:
+      name: nginx
+      state: reloaded