postgresql-backup: move to own services

This way the `startAt` setting is only set once.

services/postgresql-backup.nix

@@ -0,0 +1,19 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.my.services.postgresql-backup;
+in {
+  options.my.services.postgresql-backup = {
+    enable = mkEnableOption "Backup SQL databases";
+  };
+
+  config = mkIf cfg.enable {
+    services.postgresqlBackup = {
+      enable = true;
+      # Borg backup starts at midnight so create DB dump just before
+      startAt = "*-*-* 23:30:00";
+    };
+  };
+}