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>

README.md

@@ -13,7 +13,6 @@ NixOS Flake-based configuration repository for a homelab infrastructure. All hos
 | `monitoring01` | Prometheus, Grafana, Loki, Tempo, Pyroscope |
 | `jelly01` | Jellyfin media server |
 | `nix-cache01` | Nix binary cache |
-| `pgdb1` | PostgreSQL |
 | `nats1` | NATS messaging |
 | `vault01` | OpenBao (Vault) secrets management |
 | `template1`, `template2` | VM templates for cloning new hosts |