pgdb1: decommission postgresql host

Remove pgdb1 host configuration and postgres service module.
The only consumer (Open WebUI on gunter) has migrated to local PostgreSQL.

Removed:
- hosts/pgdb1/ - host configuration
- services/postgres/ - service module (only used by pgdb1)
- postgres_rules from monitoring rules
- rebuild-all.sh (obsolete script)

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

services/monitoring/rules.yml

@@ -356,32 +356,6 @@ 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: jellyfin_rules
     rules:
       - alert: jellyfin_down