fix: address CodeRabbit review findings

- Use FlashType enum in clearQuiz/finalizeQuiz/unfinalizeQuiz (was raw 'success'/'error' strings)
- Catch UniqueConstraintViolationException in addLabel to handle concurrent duplicate inserts
- Wrap assignToQuiz in a transaction with PESSIMISTIC_WRITE lock to serialise concurrent assignments of the same BankQuestion

src/Controller/Backoffice/QuestionBankController.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Tvdt\Controller\Backoffice;
 
+use Doctrine\DBAL\Exception\UniqueConstraintViolationException;
 use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use Symfony\Component\HttpFoundation\Request;
@@ -215,9 +216,13 @@ class QuestionBankController extends AbstractController
 
         $exists = $season->questionLabels->exists(static fn (int $key, QuestionLabel $label): bool => $label->name === $name);
         if (!$exists) {
+            try {
                 $season->addQuestionLabel(new QuestionLabel($name));
                 $this->em->flush();
                 $this->addFlash(FlashType::Success, $this->translator->trans('Label added'));
+            } catch (UniqueConstraintViolationException) {
+                // Concurrent request already inserted the same label; treat as a no-op
+            }
         }
 
         return $this->redirectToRoute('tvdt_backoffice_question_bank', ['seasonCode' => $season->seasonCode]);

src/Controller/Backoffice/QuizController.php

@@ -293,9 +293,9 @@ class QuizController extends AbstractController
             $this->quizRepository->clearQuiz($quiz);
             $quiz->finalizedAt = null;
             $this->em->flush();
-            $this->addFlash('success', $this->translator->trans('Quiz cleared and no longer finalized'));
+            $this->addFlash(FlashType::Success, $this->translator->trans('Quiz cleared and no longer finalized'));
         } catch (ErrorClearingQuizException) {
-            $this->addFlash('error', $this->translator->trans('Error clearing quiz'));
+            $this->addFlash(FlashType::Danger, $this->translator->trans('Error clearing quiz'));
         }
 
         return $this->redirectToRoute('tvdt_backoffice_quiz', ['seasonCode' => $quiz->season->seasonCode, 'quiz' => $quiz->id]);
@@ -316,7 +316,7 @@ class QuizController extends AbstractController
         } elseif (!$quiz->isFinalized()) {
             $quiz->finalizedAt = new DateTimeImmutable();
             $this->em->flush();
-            $this->addFlash('success', $this->translator->trans('Quiz finalized'));
+            $this->addFlash(FlashType::Success, $this->translator->trans('Quiz finalized'));
         }
 
         return $this->redirectToRoute('tvdt_backoffice_quiz', ['seasonCode' => $quiz->season->seasonCode, 'quiz' => $quiz->id]);
@@ -339,7 +339,7 @@ class QuizController extends AbstractController
         } else {
             $quiz->finalizedAt = null;
             $this->em->flush();
-            $this->addFlash('success', $this->translator->trans('Quiz is no longer finalized'));
+            $this->addFlash(FlashType::Success, $this->translator->trans('Quiz is no longer finalized'));
         }
 
         return $this->redirectToRoute('tvdt_backoffice_quiz', ['seasonCode' => $quiz->season->seasonCode, 'quiz' => $quiz->id]);

src/Service/QuestionBankService.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Tvdt\Service;
 
+use Doctrine\DBAL\LockMode;
 use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Component\ObjectMapper\ObjectMapperInterface;
 use Tvdt\Entity\Answer;
@@ -37,6 +38,10 @@ final readonly class QuestionBankService
             throw new QuizLockedException();
         }
 
+        $this->entityManager->wrapInTransaction(function () use ($bankQuestion, $quiz): void {
+            // Pessimistic write lock serialises concurrent assignment attempts for the same BankQuestion
+            $this->entityManager->lock($bankQuestion, LockMode::PESSIMISTIC_WRITE);
+
             if (!$bankQuestion->canBeAssigned() || $bankQuestion->isUsedInQuiz($quiz)) {
                 throw new BankQuestionAlreadyUsedException();
             }
@@ -65,6 +70,7 @@ final readonly class QuestionBankService
 
             $this->entityManager->persist($question);
             $this->entityManager->flush();
+        });
     }
 
     /**