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/discord-gateway/src/modules/voice-recording/player.ts

@@ -18,7 +18,7 @@ export class DiscordPlayer {
   private connection: VoiceConnection | null = null;
   private owner: DiscordPlayerOwner = "none";
   private resource: AudioResource | null = null;
-  private musicVolume = 1;
+  private musicVolume = 0.3;
   private idleCallback: (() => void) | null = null;
   /** Set before manual stop() calls to distinguish from natural track end. */
   private manualStop = false;