feat: Introduce FileVisibility enum and refactor FileUploadService to use it instead of magic strings for file access levels.

apps/mobile/packages/domain/lib/src/core/services/api_services/file_visibility.dart

@@ -0,0 +1,14 @@
+/// Represents the accessibility level of an uploaded file.
+enum FileVisibility {
+  /// File is accessible only to authenticated owners/authorized users.
+  private('private'),
+
+  /// File is accessible publicly via its URL.
+  public('public');
+
+  /// Creates a [FileVisibility].
+  const FileVisibility(this.value);
+
+  /// The string value expected by the backend.
+  final String value;
+}