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/meta/main.yml

@@ -0,0 +1,5 @@
+---
+# No meta dependencies: shared services (postgres) are deployed once via
+# playbook role ordering (tagged 'infra'), not re-run on every deploy of
+# this role. See playbooks/*.yml.
+dependencies: []