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.

database/migrations/2026_09_20_140000_add_primary_category_support_to_artworks.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        if (Schema::hasTable('artworks') && ! Schema::hasColumn('artworks', 'primary_category_id')) {
+            Schema::table('artworks', function (Blueprint $table): void {
+                $table->unsignedBigInteger('primary_category_id')->nullable()->after('user_id');
+                $table->index('primary_category_id');
+                $table->foreign('primary_category_id')
+                    ->references('id')
+                    ->on('categories')
+                    ->nullOnDelete();
+            });
+        }
+
+        if (Schema::hasTable('uploads') && ! Schema::hasColumn('uploads', 'secondary_category_ids')) {
+            Schema::table('uploads', function (Blueprint $table): void {
+                $table->json('secondary_category_ids')->nullable()->after('category_id');
+            });
+        }
+    }
+
+    public function down(): void
+    {
+        if (Schema::hasTable('artworks') && Schema::hasColumn('artworks', 'primary_category_id')) {
+            Schema::table('artworks', function (Blueprint $table): void {
+                $table->dropForeign(['primary_category_id']);
+                $table->dropColumn('primary_category_id');
+            });
+        }
+
+        if (Schema::hasTable('uploads') && Schema::hasColumn('uploads', 'secondary_category_ids')) {
+            Schema::table('uploads', function (Blueprint $table): void {
+                $table->dropColumn('secondary_category_ids');
+            });
+        }
+    }
+};