Symfony controller to register and activate users

I've been in an interview some time ago. They objected my Symfony API code with following line:

Too much code in controller actions. Can be moved to services to keep the business logic apart from the representation. Also violating single responsibility.

I have been trying to improve my code since then. Trying to bring SOLID in my code design and writing services for different tasks.

Now I am working on an app where I have written a controller. It still looks bulky even though it is just using services.

There are two methods:

1. Register, to register user, add user to database and send activation code to their email
2. Activate, get activation code from URL and activate the user or show error.

I want to know if there is any room for improvement here.

namespace App\Controller;

use App\Entity\User;
use App\Form\UserType;
use App\Serializer\FormErrorSerializer;
use App\Utils\ActivationKeyManager;
use Doctrine\ORM\EntityManagerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Generator\UrlGenerator;
use Symfony\Component\Security\Core\Encoder\UserPasswordEncoderInterface;
use Symfony\Component\Translation\TranslatorInterface;

/**
 * Class AuthController
 * @package App\Controller
 */
class AuthController extends AbstractController
{
    /**
     * @param Request $request
     * @param TranslatorInterface $translator
     * @param EntityManagerInterface $em
     * @param UserPasswordEncoderInterface $encoder
     * @param FormErrorSerializer $formErrorSerializer
     * @param ActivationKeyManager $activationKeyManager
     * @param \Swift_Mailer $mailer
     * @return JsonResponse
     */
    public function register(
        Request $request,
        TranslatorInterface $translator,
        EntityManagerInterface $em,
        UserPasswordEncoderInterface $encoder,
        FormErrorSerializer $formErrorSerializer,
        ActivationKeyManager $activationKeyManager,
        \Swift_Mailer $mailer
    ) : JsonResponse {
        $form = $this->createForm(UserType::class);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            /**
             * @var User $user
             */
            $user = $form->getData();
            $encoded = $encoder->encodePassword($user, $user->getPlainPassword());
            $user->setPassword($encoded);
            $em->persist( $user );
            $em->flush();

            $activationLink = $this->generateUrl(
                'activate',
                [
                    'code' => $activationKeyManager->getKey($user),
                    'user' => $user->getId()
                ],
                UrlGenerator::ABSOLUTE_URL
            );
            $message = (new \Swift_Message($translator->trans('Activate your account')))
                ->setFrom($this->getParameter('admin_email'))
                ->setTo($user->getEmail())
                ->setBody(
                    $this->renderView(
                        'emails/registration.txt.twig',
                        ['activationLink' => $activationLink]
                    ),
                    'text/plain'
                );

            $mailer->send($message);

            return new JsonResponse(['status' => 'success' , 'details' => $translator->trans('User has been created successfully.')]);
        }

        return new JsonResponse(['status' => 'error' , 'details' => $formErrorSerializer->convertFormToArray($form)], Response::HTTP_BAD_REQUEST);
    }

    /**
     * @param Request $request
     * @param TranslatorInterface $translator
     * @param EntityManagerInterface $em
     * @param ActivationKeyManager $activationKeyManager
     * @return JsonResponse
     */
    public function activate(
        Request $request,
        TranslatorInterface $translator,
        EntityManagerInterface $em,
        ActivationKeyManager $activationKeyManager
    ) {
        $user_id = $request->get('user');
        $activationCode = $request->get('code');

        if (!$user_id || !$activationCode) {
            return $this->sendInvalidActionCodeResponse($translator);
        }
        /**
         * @var User $user
         */
        $user = $em->getRepository(User::class)->find($user_id);
        if (!$user) {
            return $this->sendInvalidActionCodeResponse($translator);
        }

        if ($activationKeyManager->keyIsValid($user, $activationCode)) {
            $user->setActive(true);
            $em->flush($user);
            return new JsonResponse(['status' => 'success' , 'details' => $translator->trans('Your account has been activated.')]);
        } else {
            return $this->sendInvalidActionCodeResponse($translator);
        }
    }

    /**
     * @param TranslatorInterface $translator
     * @return JsonResponse
     */
    private function sendInvalidActionCodeResponse(TranslatorInterface $translator)
    {
        return new JsonResponse(['status' => 'error' , 'details' => $translator->trans('The provided activation code is not valid.')], Response::HTTP_BAD_REQUEST);
    }
}