fix: correct lock guards, flush batching, and clearQuiz atomicity in question bank

- syncUsagesAfterEdit: use isLocked() instead of !isFinalized() to avoid
  syncing quizzes with started candidates (would have destroyed GivenAnswer records)
- syncToQuiz action: use isLocked() instead of hasStartedCandidates() to block
  syncing into finalized quizzes that have no started candidates
- QuestionBankService::syncToQuiz: remove internal flush(); callers now
  flush once (syncUsagesAfterEdit after the loop, syncToQuiz action after the call)
- QuizRepository::clearQuiz: delete BankQuestionUsage rows and reset
  finalized_at inside the transaction (previously orphaned usages blocked
  bank question reassignment; finalizedAt reset was a separate non-atomic flush)
- QuizController::finalizeQuiz: add flash when quiz is already finalized
- QuestionBankController::delete: block deletion when any usage references a locked quiz
- BankQuestion::__toString: remove dead null-coalescing on non-nullable string
- SeasonController::addBlankQuiz: align form field with UploadQuizFormType
  (add translation_domain: false, use translator for label)

src/Service/QuestionBankService.php

@@ -97,8 +97,6 @@ final readonly class QuestionBankService
             $answer = $this->objectMapper->map($bankAnswer, Answer::class);
             $question->addAnswer($answer);
         }
-
-        $this->entityManager->flush();
     }
 
     /** Remove the quiz copy created by this usage and delete the usage record. */