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.caddy.j2

@@ -0,0 +1,26 @@
+# Grafana Metrics Dashboard
+{{ grafana_domain }} {
+    reverse_proxy http://{{ grafana_listen_address }}:{{ grafana_listen_port }} {
+        header_up Host {host}
+        header_up X-Real-IP {remote_host}
+        header_up X-Forwarded-Proto https
+        header_up X-Forwarded-For {remote_host}
+        header_up X-Forwarded-Host {host}
+    }
+
+    # Security headers
+    header {
+        X-Frame-Options SAMEORIGIN
+        X-Content-Type-Options nosniff
+        X-XSS-Protection "1; mode=block"
+        Referrer-Policy strict-origin-when-cross-origin
+        Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
+    }
+
+    # Logging
+    log {
+        output file {{ caddy_log_dir }}/grafana.log
+        level INFO
+        format json
+    }
+}