add element persistence

backend/database/migrations/2026_08_08_000000_create_elements_table.php

@@ -0,0 +1,35 @@
+<?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('elements', function (Blueprint $table): void {
+            $table->id();
+            $table->foreignId('set_id')
+                ->constrained('sets')
+                ->restrictOnDelete();
+            $table->string('name');
+            $table->string('kind');
+            $table->foreignId('parent_element_id')
+                ->nullable()
+                ->constrained('elements')
+                ->restrictOnDelete();
+            $table->unsignedInteger('position');
+            $table->index([
+                'set_id',
+                'parent_element_id',
+                'position',
+            ]);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('elements');
+    }
+};