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/hooks/use-media.ts

@@ -38,10 +38,6 @@ export function useMediaStop() {
   return useMediaAction(() => mediaApi.stop());
 }
 
-export function useMediaVolume() {
-  return useMediaAction((volume: number) => mediaApi.volume(volume));
-}
-
 /** Subscribe to WS media_state events to keep cache fresh */
 export function useMediaWsSync(ws: WsHook) {
   const { mutate } = useSWRConfig();