Add monitoring profile: Grafana + Prometheus + Loki + Alloy

Native NixOS modules under my.profiles.monitoring, enabled on jupiter.
Grafana provisions datasources/dashboards/alert rules from the
grafana-content flake input (rechberg dashboards repo). Prometheus
scrapes host + stack (90d), Loki+Alloy ship the systemd journal (90d).
Grafana LAN-only :3000, anonymous viewer, admin password + secret_key
via /etc/grafana file providers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CZJkwCnSbykq9rRTTeHc6b

machines/jupiter/environments.nix

@@ -22,6 +22,7 @@ in
     jellyfin.enable = true;
     jellyseerr.enable = true;
     immich.enable = true;
+    monitoring.enable = true;
     development.enable = true;
     home-assistant.enable = true;