monitoring: bracket IPv6 listenAddress for node_exporter

The NixOS module concatenates listenAddress and port as `${a}:${p}`,
so "::" became ":::9100" and node_exporter rejected it ("too many
colons in address"). Use "[::]" so the result is "[::]:9100".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

modules/monitoring-node-exporter.nix

@@ -4,7 +4,7 @@
   services.prometheus.exporters.node = {
     enable = true;
     port = 9100;
-    listenAddress = "::";
+    listenAddress = "[::]";
     enabledCollectors = [ "systemd" ];
   };