feat(media): default music volume 30% instead of 100%

Volume play music terlalu besar buat user — default sekarang 0.3 (30%)
di semua layer: player gateway (musicVolume=0.3), backend state/schema
(default 0.3), dan UI slider (fallback 0.3). User tetap bisa naikin
manual via slider volume di dashboard.

services/frontend/src/components/media/music-player.tsx

@@ -147,8 +147,8 @@ export function MusicPlayer({ ws, initialData }: MusicPlayerProps) {
             <Volume2 className="size-4 text-muted-foreground" />
             <Slider
               className="w-24"
-              defaultValue={[mediaState?.musicVolume ?? 0.5]}
-              value={[mediaState?.musicVolume ?? 0.5]}
+              defaultValue={[mediaState?.musicVolume ?? 0.3]}
+              value={[mediaState?.musicVolume ?? 0.3]}
               onValueChange={handleVolume}
               min={0}
               max={1}