Add unique index constraint for quiz_candidate with soft delete support

- Updated migration to include a unique index on `quiz_candidate` table that excludes soft-deleted records.
- Adjusted `QuizCandidate` entity to reflect the new unique constraint with `deleted_at` condition.

src/Entity/QuizCandidate.php

@@ -14,7 +14,7 @@ use Tvdt\Repository\QuizCandidateRepository;
 
 #[Gedmo\SoftDeleteable]
 #[ORM\Entity(repositoryClass: QuizCandidateRepository::class)]
-#[ORM\UniqueConstraint(columns: ['candidate_id', 'quiz_id'])]
+#[ORM\UniqueConstraint(columns: ['candidate_id', 'quiz_id'], options: ['where' => 'deleted_at IS NULL'])]
 class QuizCandidate
 {
     use SoftDeleteableEntity;