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

apps/mobile/packages/core/lib/src/services/api_service/core_api_services/file_upload/file_upload_service.dart

@@ -10,18 +10,18 @@ class FileUploadService extends BaseCoreService {
   /// Uploads a file with optional visibility and category.
   ///
   /// [filePath] is the local path to the file.
-  /// [visibility] can be 'public' or 'private'.
+  /// [visibility] can be [FileVisibility.public] or [FileVisibility.private].
   /// [category] is an optional metadata field.
   Future<ApiResponse> uploadFile({
     required String filePath,
     required String fileName,
-    String visibility = 'private',
+    FileVisibility visibility = FileVisibility.private,
     String? category,
   }) async {
     return action(() async {
       final FormData formData = FormData.fromMap(<String, dynamic>{
         'file': await MultipartFile.fromFile(filePath, filename: fileName),
-        'visibility': visibility,
+        'visibility': visibility.value,
         if (category != null) 'category': category,
       });