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/backend/src/modules/media/media.schema.ts

@@ -6,7 +6,7 @@ export const mediaQueueSchema = z.object({
 });
 
 export const mediaVolumeSchema = z.object({
-  volume: z.number().min(0).max(1).default(1.0),
+  volume: z.number().min(0).max(1).default(0.3),
 });
 
 export type MediaQueueInput = z.infer<typeof mediaQueueSchema>;