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/utils/ios_container_paths.dart

@@ -0,0 +1,32 @@
+final _iosSandboxRoot = RegExp(
+  r'^(?:/private)?/var/mobile/Containers/Data/Application/[A-F0-9\-]+(?=/)'
+  r'|^/[^\n]+/Library/Developer/CoreSimulator/Devices/[A-F0-9\-]+'
+  r'/data/Containers/Data/Application/[A-F0-9\-]+(?=/)',
+  caseSensitive: false,
+);
+
+/// Restores an app-owned path after iOS relocates the app's data container.
+/// Bookmarked external folders must be resolved through their bookmark instead.
+String rebaseIosSandboxPath(String path, String documentsPath) {
+  final current = _iosSandboxRoot.firstMatch(documentsPath);
+  final previous = _iosSandboxRoot.firstMatch(path);
+  if (current == null ||
+      previous == null ||
+      documentsPath.substring(current.end) != '/Documents') {
+    return path;
+  }
+
+  final suffix = path.substring(previous.end);
+  const appDirectories = [
+    '/Documents',
+    '/Library/Application Support',
+    '/Library/Caches',
+  ];
+  if (!appDirectories.any(
+        (directory) => suffix == directory || suffix.startsWith('$directory/'),
+      ) ||
+      suffix.split('/').contains('..')) {
+    return path;
+  }
+  return '${documentsPath.substring(0, current.end)}$suffix';
+}