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/CollectionAiCurationService.php

@@ -140,7 +140,7 @@ class CollectionAiCurationService
                 ->first();
 
             $label = $tag?->name
-                ?: ($artwork->categories->first()?->name)
+                ?: ($artwork->resolvedPrimaryCategory()?->name)
                 ?: ($artwork->stats?->views ? 'Popular highlights' : 'Curated picks');
 
             if (! isset($themeBuckets[$label])) {
@@ -673,7 +673,7 @@ class CollectionAiCurationService
         $artworks = $this->candidateArtworks($collection, $draft, 36);
         $themes = $this->topThemes($artworks);
         $categories = $artworks
-            ->map(fn (Artwork $artwork) => $artwork->categories->first()?->name)
+            ->map(fn (Artwork $artwork) => $artwork->resolvedPrimaryCategory()?->name)
             ->filter()
             ->countBy()
             ->sortDesc();