add explicit set levels

backend/database/migrations/2026_08_07_000000_create_set_levels_table.php

@@ -0,0 +1,27 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('set_levels', function (Blueprint $table): void {
+            $table->id();
+            $table->foreignId('set_id')
+                ->constrained('sets')
+                ->cascadeOnDelete();
+            $table->string('kind');
+            $table->unsignedInteger('depth');
+            $table->unique(['set_id', 'kind']);
+            $table->unique(['set_id', 'depth']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('set_levels');
+    }
+};