feat(audio): looping per-season background music system

MusicService (looping audioplayers player, independent of SFX) driven by the
active season theme's new 'bgm' key; switches track on season change, pauses on
app background, all failures swallowed. Separate Music on/off toggle in Settings
(persisted, independent of SFX). Season packs carry bgm keys (menu/season_001/
season_002), manifest regenerated. Assets slot assets/audio/bgm/ ready — drop in
menu.mp3/season_001.mp3/season_002.mp3 (CC0) and it plays; silent until then.
180 tests green, analyze clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/game/models/season.dart

@@ -11,6 +11,7 @@ class SeasonTheme {
     this.particleType = 'petals',
     this.tilePalette,
     this.boardTint,
+    this.bgm = 'menu',
   });
 
   factory SeasonTheme.fromJson(Map<String, dynamic> json) => SeasonTheme(
@@ -25,6 +26,7 @@ class SeasonTheme {
             ? [for (final c in json['tilePalette'] as List) (c as num).toInt()]
             : null,
         boardTint: json['boardTint'] as int?,
+        bgm: (json['bgm'] as String?) ?? 'menu',
       );
 
   /// Season 1 "First Bloom": deep navy dusk.
@@ -48,6 +50,10 @@ class SeasonTheme {
   /// Optional board background override.
   final int? boardTint;
 
+  /// Looping background-music track key, resolved to `assets/audio/bgm/KEY.mp3`.
+  /// Remote seasons reuse bundled track keys (no remote audio download).
+  final String bgm;
+
   Map<String, dynamic> toJson() => {
         'tileSet': tileSet,
         'background': background,
@@ -56,6 +62,7 @@ class SeasonTheme {
         'particleType': particleType,
         if (tilePalette != null) 'tilePalette': tilePalette,
         if (boardTint != null) 'boardTint': boardTint,
+        'bgm': bgm,
       };
 }