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/monitoring/rules.yml

@@ -115,6 +115,14 @@ groups:
         annotations:
           summary: "NSD not running on {{ $labels.instance }}"
           description: "NSD has been down on {{ $labels.instance }} more than 5 minutes."
+      - alert: unbound_low_cache_hit_ratio
+        expr: (rate(unbound_cache_hits_total[5m]) / (rate(unbound_cache_hits_total[5m]) + rate(unbound_cache_misses_total[5m]))) < 0.5
+        for: 15m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Low DNS cache hit ratio on {{ $labels.instance }}"
+          description: "Unbound cache hit ratio is below 50% on {{ $labels.instance }}."
   - name: http_proxy_rules
     rules:
       - alert: caddy_down
@@ -151,6 +159,14 @@ groups:
         annotations:
           summary: "NATS not running on {{ $labels.instance }}"
           description: "NATS has been down on {{ $labels.instance }} more than 5 minutes."
+      - alert: nats_slow_consumers
+        expr: nats_core_slow_consumer_count > 0
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "NATS has slow consumers on {{ $labels.instance }}"
+          description: "NATS has {{ $value }} slow consumers on {{ $labels.instance }}."
   - name: nix_cache_rules
     rules:
       - alert: build_flakes_service_not_active_recently
@@ -364,3 +380,83 @@ groups:
         annotations:
           summary: "Proxmox VM {{ $labels.id }} is stopped"
           description: "Proxmox VM {{ $labels.id }} ({{ $labels.name }}) has onboot=1 but is stopped."
+  - name: postgres_rules
+    rules:
+      - alert: postgres_down
+        expr: node_systemd_unit_state{instance="pgdb1.home.2rjus.net:9100", name="postgresql.service", state="active"} == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "PostgreSQL not running on {{ $labels.instance }}"
+          description: "PostgreSQL has been down on {{ $labels.instance }} more than 5 minutes."
+      - alert: postgres_exporter_down
+        expr: up{job="postgres"} == 0
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "PostgreSQL exporter down on {{ $labels.instance }}"
+          description: "Cannot scrape PostgreSQL metrics from {{ $labels.instance }}."
+      - alert: postgres_high_connections
+        expr: pg_stat_activity_count / pg_settings_max_connections > 0.8
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "PostgreSQL connection pool near exhaustion on {{ $labels.instance }}"
+          description: "PostgreSQL is using over 80% of max_connections on {{ $labels.instance }}."
+  - name: auth_rules
+    rules:
+      - alert: authelia_down
+        expr: node_systemd_unit_state{instance="auth01.home.2rjus.net:9100", name="authelia-auth.service", state="active"} == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Authelia not running on {{ $labels.instance }}"
+          description: "Authelia has been down on {{ $labels.instance }} more than 5 minutes."
+      - alert: lldap_down
+        expr: node_systemd_unit_state{instance="auth01.home.2rjus.net:9100", name="lldap.service", state="active"} == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "LLDAP not running on {{ $labels.instance }}"
+          description: "LLDAP has been down on {{ $labels.instance }} more than 5 minutes."
+  - name: jellyfin_rules
+    rules:
+      - alert: jellyfin_down
+        expr: up{job="jellyfin"} == 0
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Jellyfin not responding on {{ $labels.instance }}"
+          description: "Cannot scrape Jellyfin metrics from {{ $labels.instance }} for 5 minutes."
+  - name: vault_rules
+    rules:
+      - alert: openbao_down
+        expr: node_systemd_unit_state{instance="vault01.home.2rjus.net:9100", name="openbao.service", state="active"} == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "OpenBao not running on {{ $labels.instance }}"
+          description: "OpenBao has been down on {{ $labels.instance }} more than 5 minutes."
+      - alert: openbao_sealed
+        expr: vault_core_unsealed == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "OpenBao is sealed on {{ $labels.instance }}"
+          description: "OpenBao has been sealed on {{ $labels.instance }} for more than 5 minutes."
+      - alert: openbao_scrape_down
+        expr: up{job="openbao"} == 0
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Cannot scrape OpenBao metrics from {{ $labels.instance }}"
+          description: "OpenBao metrics endpoint is not responding on {{ $labels.instance }}."