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/lib/hooks/use-media-player.tsx

@@ -41,7 +41,7 @@ export function MediaPlayerProvider({ children }: { children: ReactNode }) {
   const ws = useWebSocket();
   const [state, setState] = useState<MediaState>({
     playing: false,
-    musicVolume: 0.5,
+    musicVolume: 0.3,
     current: null,
     queue: [],
   });