feat(backend): migrate production backend to Rust

Replace the Go backend with the Rust workspace and route Android/iOS through
UniFFI bindings. Include the migrated extension runtime, network, providers,
media metadata, downloads and library operations, with version 5.0.0+147.

Remove Go sources, adapters, native build selection and CI dependencies.
Build Rust unconditionally and lock the iOS Rust pod using a relative path.
Retain legacy source and migration evidence in a local ignored archive.

Validation: Android Kotlin compile and 53 native tests; Swift Rust-branch
equivalence and syntax; CocoaPods install; workflow, shell, Ruby, plist and
diff checks. Reuse the preceding 100 Rust tests, fmt/Clippy and five-ABI
build checkpoint; no new APK or iOS application build for this cleanup.

Known follow-up: URL/URLSearchParams globals are missing from the Rust JS
runtime. A controlled extension replay confirms a URL-resolution regression;
this commit does not fix that runtime gap.

lib/models/settings.dart

@@ -197,6 +197,15 @@ class AppSettings {
     this.playerMode = 'external',
   });
 
+  @JsonKey(includeFromJson: false, includeToJson: false)
+  Map<String, dynamic> get lyricsFetchOptions => {
+    'include_translation_netease': lyricsIncludeTranslationNetease,
+    'include_romanization_netease': lyricsIncludeRomanizationNetease,
+    'multi_person_word_by_word': lyricsMultiPersonWordByWord,
+    'apple_elrc_word_sync': lyricsAppleElrcWordSync,
+    'musixmatch_language': musixmatchLanguage,
+  };
+
   AppSettings copyWith({
     String? defaultService,
     String? audioQuality,