enforce element level hierarchy

backend/database/migrations/2026_08_08_000000_create_elements_table.php

@@ -10,18 +10,17 @@ return new class extends Migration
     {
         Schema::create('elements', function (Blueprint $table): void {
             $table->id();
-            $table->foreignId('set_id')
-                ->constrained('sets')
+            $table->foreignId('set_level_id')
+                ->constrained('set_levels')
                 ->restrictOnDelete();
             $table->string('name');
-            $table->string('kind');
             $table->foreignId('parent_element_id')
                 ->nullable()
                 ->constrained('elements')
                 ->restrictOnDelete();
             $table->unsignedInteger('position');
             $table->index([
-                'set_id',
+                'set_level_id',
                 'parent_element_id',
                 'position',
             ]);