feat(media): clean media api + create an entity per media type

app/Entities/Media/Transcript.php

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * @copyright  2021 Podlibre
+ * @license    https://www.gnu.org/licenses/agpl-3.0.en.html AGPL3
+ * @link       https://castopod.org/
+ */
+
+namespace App\Entities\Media;
+
+class Transcript extends BaseMedia
+{
+    protected string $type = 'transcript';
+}