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.

app/Services/CollectionLinkService.php

@@ -95,7 +95,7 @@ class CollectionLinkService
                 'label' => (string) $artwork->title,
                 'description' => collect([
                     $artwork->user?->username ? '@' . strtolower((string) $artwork->user->username) : null,
-                    $artwork->categories->first()?->contentType?->name,
+                    $artwork->resolvedPrimaryCategory()?->contentType?->name,
                 ])->filter()->join(' • ') ?: 'Published artwork',
             ])
             ->values()
@@ -343,7 +343,7 @@ class CollectionLinkService
                 'title' => (string) $entity->title,
                 'subtitle' => collect([
                     $entity->user?->username ? '@' . strtolower((string) $entity->user->username) : null,
-                    $entity->categories->first()?->contentType?->name,
+                    $entity->resolvedPrimaryCategory()?->contentType?->name,
                 ])->filter()->join(' • ') ?: 'Artwork',
                 'description' => $link->relationship_type ?: 'Linked artwork',
                 'url' => route('art.show', [