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/Studio/StudioAiCategoryMapper.php

@@ -49,8 +49,9 @@ final class StudioAiCategoryMapper
                 $selectedCategory,
                 $this->confidenceForModel($categoryScores, $selectedCategory->id),
                 $categoryScores
-                    ->reject(fn (array $row): bool => (int) $row['model']->id === (int) $selectedCategory->id)
-                    ->take(3)
+                    ->reject(fn (array $row): bool => (int) $row['model']->id === (int) $selectedCategory->id
+                        || (int) $row['model']->content_type_id !== (int) $selectedCategory->content_type_id)
+                    ->take(max(0, (int) config('categories.max_secondary_categories', 5)))
                     ->map(fn (array $row): array => $this->serializeCategory($row['model'], $row['confidence']))
                     ->all()
             ) : null,
@@ -245,6 +246,7 @@ final class StudioAiCategoryMapper
             'root_category_id' => (int) $rootCategory->id,
             'sub_category_id' => $category->parent_id ? (int) $category->id : null,
             'alternatives' => array_values($alternatives),
+            'secondary_categories' => array_values($alternatives),
         ];
     }
 }