feat(media): loop mode + high-quality OggOpus music playback

- Loop: toggle via POST /api/media/loop → COMMAND_MEDIA_LOOP; gateway
  replays finished music track on natural end (queue untouched); status
  payload exposes loop flag; FE tombol Loop di music-player + mini-player.
- Kualitas suara: music playback sekarang di-transcode sekali via ffmpeg ke
  OggOpus 48kHz stereo 192kbps dengan volume di-bake ke encode — menghindari
  double lossy encode (inlineVolume) yang bikin suara buram. Screen share
  tetap pakai jalur lama.
- Backend: MediaState.loop, setLoop service, route + schema validation.

services/discord-gateway/src/modules/command-handler/handler-registry.ts

@@ -1,6 +1,7 @@
 import {
   COMMAND_GUILDS_LIST,
   COMMAND_GUILDS_TEXT_CHANNELS,
+  COMMAND_MEDIA_LOOP,
   COMMAND_MEDIA_QUEUE,
   COMMAND_MEDIA_SKIP,
   COMMAND_MEDIA_STOP,
@@ -69,6 +70,7 @@ export function createHandlerRegistry(
   registry.set(COMMAND_MEDIA_VOLUME, (cmd) =>
     mediaHandler.handleMediaVolume(cmd),
   );
+  registry.set(COMMAND_MEDIA_LOOP, (cmd) => mediaHandler.handleMediaLoop(cmd));
 
   // Guild commands
   registry.set(COMMAND_GUILDS_LIST, (cmd) =>