feat(file): conceal file resource existence from other users

- feat: cross-user file access returns not-found instead of
  permission-denied
- fix: repository delete now only affects active rows and returns
  ErrNotFound when no row changes; repeated deletes yield not-found
- feat: parent directory operations (list, upload, create-dir, move)
  conceal ownership of other user's directories
- test: update handler, service, repository, and integration tests to
  assert not-found behavior for cross-user and missing file operations

internal/repository/file.go

@@ -115,9 +115,15 @@ func (r *fileRepository) Update(ctx context.Context, file *model.File) error {
 }
 
 func (r *fileRepository) Delete(ctx context.Context, id string) error {
-	result := r.db.WithContext(ctx).Model(&model.File{}).Where("id = ?", id).Update("status", model.StatusUserDeleted)
+	result := r.db.WithContext(ctx).
+		Model(&model.File{}).
+		Where("id = ? AND status = ?", id, model.StatusActive).
+		Update("status", model.StatusUserDeleted)
 	if result.Error != nil {
 		return result.Error
 	}
+	if result.RowsAffected == 0 {
+		return model.ErrNotFound
+	}
 	return nil
 }