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/services/sqlite_helpers.dart

@@ -50,6 +50,7 @@ Future<Database> openAppDatabase(
   required Future<void> Function(Database db, int version) onCreate,
   required Future<void> Function(Database db, int oldVersion, int newVersion)
   onUpgrade,
+  Future<void> Function(Database db)? onOpen,
   bool foreignKeys = false,
   bool incrementalAutoVacuum = true,
 }) async {
@@ -94,6 +95,7 @@ Future<Database> openAppDatabase(
     },
     onCreate: onCreate,
     onUpgrade: onUpgrade,
+    onOpen: onOpen,
   );
 }