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.service.ts

@@ -6,7 +6,6 @@ import {
   COMMAND_MEDIA_QUEUE,
   COMMAND_MEDIA_SKIP,
   COMMAND_MEDIA_STOP,
-  COMMAND_MEDIA_VOLUME,
   MEDIA_STATUS_KEY,
 } from "../../shared/index.js";
 import { publishCommand, readRedisStatus } from "../../shared/redis/index.js";
@@ -149,20 +148,3 @@ export async function stop(): Promise<MediaState> {
     "stop",
   );
 }
-
-/**
- * Set volume via Redis command to discord-gateway.
- */
-export async function setVolume(volume: number): Promise<MediaState> {
-  logger.info({ volume }, "setVolume called");
-  return tryCommandThenFallback(
-    () =>
-      publishCommand<MediaState>(
-        COMMAND_MEDIA_VOLUME,
-        { volume },
-        DEFAULT_COMMAND_TIMEOUT_MS,
-      ),
-    () => readStatusFallback(),
-    "setVolume",
-  );
-}