nixos-config/services/media.nix

{ config, lib, ... }:
let
  inherit (lib)
    mkIf
  ;

  mediaServices = builtins.attrValues {
    inherit (config.my.services)
      jellyfin
      transmission
    ;
  };
  needed = builtins.any (service: service.enable) mediaServices;
in
{
  config.users.groups.media = mkIf needed { };
}