refactor(media): remove volume control from FE & BE

Volume sudah di-set default 0.3 di gateway (suara kecil saat play), dan
user bisa naikin sendiri di Discord (command media:volume) — jadi kontrol
volume lewat dashboard tak perlu. Hapus:
- BE: POST /api/media/volume route, mediaVolumeSchema, setVolume service
- FE: useMediaVolume hook, mediaApi.volume, slider volume di music-player
  + mini-player, field volume/setVolume di MediaPlayerProvider
Pertahankan COMMAND_MEDIA_VOLUME di gateway (masih dipakai command DC)
dan mic volume (terpisah, tetap di voice page).

services/frontend/src/lib/api/media.ts

@@ -7,6 +7,4 @@ export const mediaApi = {
     api.post<MediaState>("/api/media/queue", { source, mode }),
   skip: () => api.post<MediaState>("/api/media/skip", {}),
   stop: () => api.post<MediaState>("/api/media/stop", {}),
-  volume: (volume: number) =>
-    api.post<MediaState>("/api/media/volume", { volume }),
 };