Add metrics monitoring stack with VictoriaMetrics, Grafana, and node_exporter

Implement complete monitoring infrastructure following rick-infra principles:

Components:
- VictoriaMetrics: Prometheus-compatible TSDB (7x less RAM usage)
- Grafana: Visualization dashboard with Authentik OAuth/OIDC integration
- node_exporter: System metrics collection (CPU, memory, disk, network)

Architecture:
- All services run as native systemd binaries (no containers)
- localhost-only binding for security
- Grafana uses native OAuth integration with Authentik (not forward_auth)
- Full systemd security hardening enabled
- Proxied via Caddy at metrics.jnss.me with HTTPS

Role Features:
- Unified metrics role (single role for complete stack)
- Automatic role mapping via Authentik groups:
  - authentik Admins OR grafana-admins -> Admin access
  - grafana-editors -> Editor access
  - All others -> Viewer access
- VictoriaMetrics auto-provisioned as default Grafana datasource
- 12-month metrics retention by default
- Comprehensive documentation included

Security:
- OAuth/OIDC SSO via Authentik
- All metrics services bind to 127.0.0.1 only
- systemd hardening (NoNewPrivileges, ProtectSystem, etc.)
- Grafana accessible only via Caddy HTTPS proxy

Documentation:
- roles/metrics/README.md: Complete role documentation
- docs/metrics-deployment-guide.md: Step-by-step deployment guide

Configuration:
- Updated rick-infra.yml to include metrics deployment
- Grafana port set to 3001 (Gitea uses 3000)
- Ready for multi-host expansion (designed for future node_exporter deployment to production hosts)

roles/metrics/templates/grafana.service.j2

@@ -0,0 +1,36 @@
+[Unit]
+Description=Grafana visualization platform
+Documentation=https://grafana.com/docs/
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User={{ grafana_user }}
+Group={{ grafana_group }}
+
+WorkingDirectory=/opt/grafana
+ExecStart=/opt/grafana/bin/grafana-server \
+  --config=/etc/grafana/grafana.ini \
+  --homepath=/opt/grafana
+
+Restart=on-failure
+RestartSec=5s
+
+# Security hardening
+{% if grafana_systemd_security %}
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=strict
+ProtectHome=true
+ReadWritePaths={{ grafana_data_dir }} {{ grafana_logs_dir }}
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectControlGroups=true
+RestrictRealtime=true
+RestrictNamespaces=true
+LockPersonality=true
+{% endif %}
+
+[Install]
+WantedBy=multi-user.target