okarin: Add postgresql backup

machines/okarin/configuration.nix

@@ -34,4 +34,14 @@
     80
     443
   ];
+
+  services.postgresqlBackup = {
+    enable = true;
+    startAt = [ ]; # triggered by restic system backup
+    location = "/var/lib/postgresql-backup";
+  };
+  systemd.services.restic-backups-system = {
+    after = [ "postgresqlBackup.service" ];
+    wants = [ "postgresqlBackup.service" ];
+  };
 }