public function up(): void
{
    Schema::table('users', function (Blueprint $table) {
        $table->foreignId('upline_id')->nullable()->after('role')->constrained('users')->onDelete('set null');
    });
}

public function down(): void
{
    Schema::table('users', function (Blueprint $table) {
        $table->dropForeign(['upline_id']);
        $table->dropColumn('upline_id');
    });
}