feat(backend): add IDHS as fallback link resolver when SongLink fails

lib/models/settings.dart

@@ -33,6 +33,7 @@ class AppSettings {
   final String locale;
   final bool enableLossyOption;
   final String lossyFormat;
+  final String lossyBitrate; // e.g., 'mp3_320', 'mp3_256', 'mp3_192', 'mp3_128', 'opus_128', 'opus_96', 'opus_64'
   final String lyricsMode;
 
   const AppSettings({
@@ -65,6 +66,7 @@ class AppSettings {
     this.locale = 'system',
     this.enableLossyOption = false,
     this.lossyFormat = 'mp3',
+    this.lossyBitrate = 'mp3_320',
     this.lyricsMode = 'embed',
   });
 
@@ -99,6 +101,7 @@ class AppSettings {
     String? locale,
     bool? enableLossyOption,
     String? lossyFormat,
+    String? lossyBitrate,
     String? lyricsMode,
   }) {
     return AppSettings(
@@ -131,6 +134,7 @@ class AppSettings {
       locale: locale ?? this.locale,
       enableLossyOption: enableLossyOption ?? this.enableLossyOption,
       lossyFormat: lossyFormat ?? this.lossyFormat,
+      lossyBitrate: lossyBitrate ?? this.lossyBitrate,
       lyricsMode: lyricsMode ?? this.lyricsMode,
     );
   }

lib/models/settings.g.dart

@@ -38,6 +38,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   locale: json['locale'] as String? ?? 'system',
   enableLossyOption: json['enableLossyOption'] as bool? ?? false,
   lossyFormat: json['lossyFormat'] as String? ?? 'mp3',
+  lossyBitrate: json['lossyBitrate'] as String? ?? 'mp3_320',
   lyricsMode: json['lyricsMode'] as String? ?? 'embed',
 );
 
@@ -72,5 +73,6 @@ Map<String, dynamic> _$AppSettingsToJson(AppSettings instance) =>
       'locale': instance.locale,
       'enableLossyOption': instance.enableLossyOption,
       'lossyFormat': instance.lossyFormat,
+      'lossyBitrate': instance.lossyBitrate,
       'lyricsMode': instance.lyricsMode,
     };