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.

migrations/Version20260524121119.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260524121119 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add unique constraint for quiz_candidate with deleted_at filter';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // this up() migration is auto-generated, please modify it to your needs
+        $this->addSql('DROP INDEX uniq_ced2ffa291bd8781853cd175');
+        $this->addSql('CREATE UNIQUE INDEX UNIQ_CED2FFA291BD8781853CD175 ON quiz_candidate (candidate_id, quiz_id) WHERE deleted_at IS NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        // this down() migration is auto-generated, please modify it to your needs
+        $this->addSql('DROP INDEX UNIQ_CED2FFA291BD8781853CD175');
+        $this->addSql('CREATE UNIQUE INDEX uniq_ced2ffa291bd8781853cd175 ON quiz_candidate (candidate_id, quiz_id)');
+    }
+}