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 }),
 };

services/frontend/src/lib/hooks/use-media-player.tsx

@@ -20,8 +20,6 @@ interface MediaPlayerContextValue {
   current: MediaItem | null;
   /** Upcoming queue */
   queue: MediaItem[];
-  /** Current volume [0-1] */
-  volume: number;
   /** True while a mutation is in flight */
   pending: boolean;
 
@@ -29,8 +27,6 @@ interface MediaPlayerContextValue {
   skip: () => void;
   /** Stop playback */
   stop: () => void;
-  /** Set volume [0-1] */
-  setVolume: (vol: number) => void;
   /** Queue a URL for playback */
   queueUrl: (url: string) => void;
 }
@@ -96,17 +92,6 @@ export function MediaPlayerProvider({ children }: { children: ReactNode }) {
       .finally(() => setPending(false));
   }, []);
 
-  const setVolume = useCallback((vol: number) => {
-    mediaApi
-      .volume(vol)
-      .then((data) => {
-        if (data) setState(data as MediaState);
-      })
-      .catch(() => {
-        // ignore
-      });
-  }, []);
-
   const queueUrl = useCallback((url: string) => {
     setPending(true);
     mediaApi
@@ -126,11 +111,9 @@ export function MediaPlayerProvider({ children }: { children: ReactNode }) {
         playing: state.playing,
         current: state.current,
         queue: state.queue,
-        volume: state.musicVolume,
         pending,
         skip,
         stop,
-        setVolume,
         queueUrl,
       }}
     >