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>

terraform/vault/secrets.tf

@@ -92,6 +92,13 @@ locals {
       auto_generate = false
       data          = { token = var.actions_token_1 }
     }
+
+    # Prometheus OpenBao token for scraping metrics
+    # Token is created by vault_token.prometheus_metrics in policies.tf
+    "hosts/monitoring01/openbao-token" = {
+      auto_generate = false
+      data          = { token = vault_token.prometheus_metrics.client_token }
+    }
   }
 }