add explicit set levels

backend/app/Set/SetLevel.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Set;
+
+final readonly class SetLevel
+{
+    public function __construct(
+        private int $id,
+        private Set $set,
+        private string $kind,
+        private int $depth,
+    ) {}
+
+    public function getId(): int
+    {
+        return $this->id;
+    }
+
+    public function getSet(): Set
+    {
+        return $this->set;
+    }
+
+    public function getKind(): string
+    {
+        return $this->kind;
+    }
+
+    public function getDepth(): int
+    {
+        return $this->depth;
+    }
+}