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>;

services/backend/src/modules/media/media.service.ts

@@ -44,7 +44,7 @@ const DEFAULT_COMMAND_TIMEOUT_MS = 5000;
 const DEFAULT_STATE: MediaState = {
   playing: false,
   activeMode: null,
-  musicVolume: 1.0,
+  musicVolume: 0.3,
   current: null,
   queue: [],
 };
@@ -65,7 +65,7 @@ function normalizeMediaState(raw: Record<string, unknown>): MediaState {
   return {
     playing,
     activeMode,
-    musicVolume: Number(raw.musicVolume ?? 1.0),
+    musicVolume: Number(raw.musicVolume ?? 0.3),
     current: (raw.current as MediaItem | null) ?? null,
     queue: (raw.queue as MediaItem[]) ?? [],
   };