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>

flake.nix

@@ -128,15 +128,6 @@
             ./hosts/nix-cache01
           ];
         };
-        pgdb1 = nixpkgs.lib.nixosSystem {
-          inherit system;
-          specialArgs = {
-            inherit inputs self;
-          };
-          modules = commonModules ++ [
-            ./hosts/pgdb1
-          ];
-        };
         nats1 = nixpkgs.lib.nixosSystem {
           inherit system;
           specialArgs = {