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/Models/Upload.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace App\Models;
 
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
 class Upload extends Model
 {
@@ -23,6 +24,7 @@ class Upload extends Model
         'title',
         'slug',
         'category_id',
+        'secondary_category_ids',
         'description',
         'tags',
         'license',
@@ -41,6 +43,7 @@ class Upload extends Model
 
     protected $casts = [
         'tags' => 'array',
+        'secondary_category_ids' => 'array',
         'nsfw' => 'boolean',
         'is_scanned' => 'boolean',
         'has_tags' => 'boolean',
@@ -48,4 +51,14 @@ class Upload extends Model
         'expires_at' => 'datetime',
         'moderated_at' => 'datetime',
     ];
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(User::class);
+    }
+
+    public function category(): BelongsTo
+    {
+        return $this->belongsTo(Category::class);
+    }
 }