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/components/media/mini-player.tsx

@@ -1,11 +1,10 @@
 "use client";
 
-import { Disc3, Music, SkipForward, Square, Volume2 } from "lucide-react";
+import { Disc3, Music, SkipForward, Square } from "lucide-react";
 import { useMediaPlayer } from "@/lib/hooks/use-media-player";
 
 export function MiniPlayer() {
-  const { playing, current, queue, volume, pending, skip, stop, setVolume } =
-    useMediaPlayer();
+  const { playing, current, queue, pending, skip, stop } = useMediaPlayer();
 
   // Nothing to show if no track is playing and nothing is queued
   if (!current && queue.length === 0) return null;
@@ -61,22 +60,6 @@ export function MiniPlayer() {
           </button>
         )}
       </div>
-
-      {/* Volume */}
-      <div className="flex items-center gap-2 shrink-0 ml-2">
-        <Volume2 className="size-3.5 text-text-secondary/60" />
-        <input
-          type="range"
-          min="0"
-          max="1"
-          step="0.05"
-          value={volume}
-          onChange={(e) => setVolume(Number(e.target.value))}
-          className="w-20 h-1 appearance-none rounded-full bg-glass-bg accent-primary cursor-pointer
-            [&::-webkit-slider-thumb]:appearance-none [&::-webkit-slider-thumb]:size-3 [&::-webkit-slider-thumb]:rounded-full [&::-webkit-slider-thumb]:bg-primary"
-          aria-label="Volume"
-        />
-      </div>
     </div>
   );
 }