monitoring: use nats exporter instead of direct scrape

NATS HTTP monitoring endpoint serves JSON, not Prometheus format.
Use the prometheus-nats-exporter which queries the NATS endpoint
and exposes proper Prometheus metrics.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/nats/default.nix

@@ -2,9 +2,14 @@
 {
   homelab.monitoring.scrapeTargets = [{
     job_name = "nats";
-    port = 8222;
+    port = 7777;
   }];
 
+  services.prometheus.exporters.nats = {
+    enable = true;
+    url = "http://localhost:8222";
+  };
+
   services.nats = {
     enable = true;
     jetstream = true;