Allow artworks to have one primary category and extra secondaries.

Keep artwork_category as membership, store a deterministic primary_category_id, and let upload plus studio pick up to five additional same-type categories without changing canonical URLs.

config/categories.php

@@ -0,0 +1,7 @@
+<?php
+
+declare(strict_types=1);
+
+return [
+    'max_secondary_categories' => max(0, (int) env('CATEGORIES_MAX_SECONDARY', 5)),
+];