nixos/epgstation: add a new option 'ffmpeg'

Closes #187120.

nixos/modules/services/video/epgstation/default.nix

···
   options.services.epgstation = {
     enable = lib.mkEnableOption (lib.mdDoc description);
 
-    package = lib.mkOption {
-      default = pkgs.epgstation;
-      type = lib.types.package;
-      defaultText = lib.literalExpression "pkgs.epgstation";
-      description = lib.mdDoc "epgstation package to use";
+    package = lib.mkPackageOptionMD pkgs "epgstation" { };
+
+    ffmpeg = lib.mkPackageOptionMD pkgs "ffmpeg" {
+      default = [ "ffmpeg-headless" ];
+      example = "pkgs.ffmpeg-full";
     };
 
     usePreconfiguredStreaming = lib.mkOption {
···
       package = lib.mkDefault pkgs.mariadb;
       ensureDatabases = [ cfg.database.name ];
       # FIXME: enable once mysqljs supports auth_socket
+      # https://github.com/mysqljs/mysql/issues/1507
+      #
       # ensureUsers = [ {
       #   name = username;
       #   ensurePermissions = { "${cfg.database.name}.*" = "ALL PRIVILEGES"; };
···
             database = cfg.database.name;
           };
 
-          ffmpeg = lib.mkDefault "${pkgs.ffmpeg-full}/bin/ffmpeg";
-          ffprobe = lib.mkDefault "${pkgs.ffmpeg-full}/bin/ffprobe";
+          ffmpeg = lib.mkDefault "${cfg.ffmpeg}/bin/ffmpeg";
+          ffprobe = lib.mkDefault "${cfg.ffmpeg}/bin/ffprobe";
 
           # for disambiguation with TypeScript files
           recordedFileExtension = lib.mkDefault ".m2ts";