monitoring: implement monitoring gaps coverage

Add exporters and scrape targets for services lacking monitoring:
- PostgreSQL: postgres-exporter on pgdb1
- Authelia: native telemetry metrics on auth01
- Unbound: unbound-exporter with remote-control on ns1/ns2
- NATS: HTTP monitoring endpoint on nats1
- OpenBao: telemetry config and Prometheus scrape with token auth
- Systemd: systemd-exporter on all hosts for per-service metrics

Add alert rules for postgres, auth (authelia + lldap), jellyfin,
vault (openbao), plus extend existing nats and unbound rules.

Add Terraform config for Prometheus metrics policy and token. The
token is created via vault_token resource and stored in KV, so no
manual token creation is needed.

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

services/nats/default.nix

@@ -1,10 +1,16 @@
 { ... }:
 {
+  homelab.monitoring.scrapeTargets = [{
+    job_name = "nats";
+    port = 8222;
+  }];
+
   services.nats = {
     enable = true;
     jetstream = true;
     serverName = "nats1";
     settings = {
+      http_port = 8222;
       accounts = {
         ADMIN = {
           users = [