refactor: remove legacy API clients, Yoinkify fallback, and unused lyrics provider

- Delete dead metadata client and extract shared types to metadata_types.go
- Remove Yoinkify download fallback from Deezer, use MusicDL only
- Clean up retired settings fields and metadataSource
- Remove dead l10n keys for retired provider
- Add migration to strip retired provider from existing users' lyrics config

lib/models/settings.dart

@@ -30,10 +30,6 @@ class AppSettings {
   final String historyViewMode;
   final String historyFilterMode;
   final bool askQualityBeforeDownload;
-  final String spotifyClientId;
-  final String spotifyClientSecret;
-  final bool useCustomSpotifyCredentials;
-  final String metadataSource;
   final bool enableLogging;
   final bool useExtensionProviders;
   final String? searchProvider;
@@ -107,10 +103,6 @@ class AppSettings {
     this.historyViewMode = 'grid',
     this.historyFilterMode = 'all',
     this.askQualityBeforeDownload = true,
-    this.spotifyClientId = '',
-    this.spotifyClientSecret = '',
-    this.useCustomSpotifyCredentials = false,
-    this.metadataSource = 'deezer',
     this.enableLogging = false,
     this.useExtensionProviders = true,
     this.searchProvider,
@@ -134,7 +126,6 @@ class AppSettings {
     this.hasCompletedTutorial = false,
     this.lyricsProviders = const [
       'lrclib',
-      'spotify_api',
       'musixmatch',
       'netease',
       'apple_music',
@@ -172,10 +163,6 @@ class AppSettings {
     String? historyViewMode,
     String? historyFilterMode,
     bool? askQualityBeforeDownload,
-    String? spotifyClientId,
-    String? spotifyClientSecret,
-    bool? useCustomSpotifyCredentials,
-    String? metadataSource,
     bool? enableLogging,
     bool? useExtensionProviders,
     String? searchProvider,
@@ -235,11 +222,6 @@ class AppSettings {
       historyFilterMode: historyFilterMode ?? this.historyFilterMode,
       askQualityBeforeDownload:
           askQualityBeforeDownload ?? this.askQualityBeforeDownload,
-      spotifyClientId: spotifyClientId ?? this.spotifyClientId,
-      spotifyClientSecret: spotifyClientSecret ?? this.spotifyClientSecret,
-      useCustomSpotifyCredentials:
-          useCustomSpotifyCredentials ?? this.useCustomSpotifyCredentials,
-      metadataSource: metadataSource ?? this.metadataSource,
       enableLogging: enableLogging ?? this.enableLogging,
       useExtensionProviders:
           useExtensionProviders ?? this.useExtensionProviders,

lib/models/settings.g.dart

@@ -32,11 +32,6 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   historyViewMode: json['historyViewMode'] as String? ?? 'grid',
   historyFilterMode: json['historyFilterMode'] as String? ?? 'all',
   askQualityBeforeDownload: json['askQualityBeforeDownload'] as bool? ?? true,
-  spotifyClientId: json['spotifyClientId'] as String? ?? '',
-  spotifyClientSecret: json['spotifyClientSecret'] as String? ?? '',
-  useCustomSpotifyCredentials:
-      json['useCustomSpotifyCredentials'] as bool? ?? false,
-  metadataSource: json['metadataSource'] as String? ?? 'deezer',
   enableLogging: json['enableLogging'] as bool? ?? false,
   useExtensionProviders: json['useExtensionProviders'] as bool? ?? true,
   searchProvider: json['searchProvider'] as String?,
@@ -65,14 +60,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       (json['lyricsProviders'] as List<dynamic>?)
           ?.map((e) => e as String)
           .toList() ??
-      const [
-        'lrclib',
-        'spotify_api',
-        'musixmatch',
-        'netease',
-        'apple_music',
-        'qqmusic',
-      ],
+      const ['lrclib', 'musixmatch', 'netease', 'apple_music', 'qqmusic'],
   lyricsIncludeTranslationNetease:
       json['lyricsIncludeTranslationNetease'] as bool? ?? false,
   lyricsIncludeRomanizationNetease:
@@ -111,10 +99,6 @@ Map<String, dynamic> _$AppSettingsToJson(
   'historyViewMode': instance.historyViewMode,
   'historyFilterMode': instance.historyFilterMode,
   'askQualityBeforeDownload': instance.askQualityBeforeDownload,
-  'spotifyClientId': instance.spotifyClientId,
-  'spotifyClientSecret': instance.spotifyClientSecret,
-  'useCustomSpotifyCredentials': instance.useCustomSpotifyCredentials,
-  'metadataSource': instance.metadataSource,
   'enableLogging': instance.enableLogging,
   'useExtensionProviders': instance.useExtensionProviders,
   'searchProvider': instance.searchProvider,