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/backend/src/modules/media/media.schema.ts

@@ -5,9 +5,4 @@ export const mediaQueueSchema = z.object({
   mode: z.enum(["music", "screen"]).default("music"),
 });
 
-export const mediaVolumeSchema = z.object({
-  volume: z.number().min(0).max(1).default(0.3),
-});
-
 export type MediaQueueInput = z.infer<typeof mediaQueueSchema>;
-export type MediaVolumeInput = z.infer<typeof mediaVolumeSchema>;