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

modules/environments/monitoring/default.nix

@@ -0,0 +1,16 @@
+# Observability stack: Grafana + Prometheus + Loki + Alloy.
+# Content (dashboards, datasources, alert rules) lives in a separate repo,
+# consumed as the `grafana-content` flake input and provisioned from the store.
+{ lib, ... }:
+{
+  imports = [
+    ./grafana.nix
+    ./prometheus.nix
+    ./loki.nix
+    ./alloy.nix
+  ];
+
+  options.my.profiles.monitoring = {
+    enable = lib.mkEnableOption "Grafana + Prometheus + Loki + Alloy observability stack";
+  };
+}