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/Http/Controllers/Api/ArtworkController.php

@@ -1,13 +1,14 @@
 <?php
+
 namespace App\Http\Controllers\Api;
 
 use App\Http\Controllers\Controller;
 use App\Http\Requests\Artworks\ArtworkCreateRequest;
 use App\Http\Resources\ArtworkListResource;
 use App\Http\Resources\ArtworkResource;
-use App\Services\ArtworkService;
-use App\Services\Artworks\ArtworkDraftService;
 use App\Models\Category;
+use App\Services\Artworks\ArtworkDraftService;
+use App\Services\ArtworkService;
 use Illuminate\Http\Request;
 use Symfony\Component\HttpFoundation\Response;
 
@@ -29,9 +30,12 @@ class ArtworkController extends Controller
         $user = $request->user();
         $data = $request->validated();
 
-        $categoryId = isset($data['category']) && ctype_digit((string) $data['category'])
-            ? (int) $data['category']
-            : null;
+        $categoryId = isset($data['primary_category_id']) && ctype_digit((string) $data['primary_category_id'])
+            ? (int) $data['primary_category_id']
+            : (isset($data['category']) && ctype_digit((string) $data['category'])
+                ? (int) $data['category']
+                : null);
+        $secondaryCategoryIds = array_values(array_map('intval', $data['secondary_category_ids'] ?? []));
 
         $result = $drafts->createDraft(
             $user,
@@ -40,6 +44,7 @@ class ArtworkController extends Controller
             $categoryId,
             (bool) ($data['is_mature'] ?? false),
             $data['group'] ?? null,
+            $secondaryCategoryIds,
         );
 
         return response()->json([