mopidy: add ffmpeg support (#468949)

nixos/modules/services/audio/mopidy.nix

@@ -55,6 +55,7 @@ in
 
       settings = lib.mkOption {
         inherit (settingsFormat) type;
+        default = { };
         example.mpd = {
           enabled = true;
           hostname = "::";

pkgs/applications/audio/mopidy/mopidy.nix

@@ -34,6 +34,7 @@ pythonPackages.buildPythonApplication (finalAttrs: {
       gst-plugins-good
       gst-plugins-ugly
       gst-plugins-rs
+      gst-libav
     ]
     ++ lib.optionals (!stdenv.hostPlatform.isDarwin) [ pipewire ];