feat(mail) : migrate Mail integration into module (back)

LST-67 (2.5) backend. Behaviour-preserving move of the IMAP mail integration
into src/Module/Mail/. All /api/mail/* routes, securities (ROLE_CLIENT still
excluded via MailAccessChecker) and the async sync are unchanged.

- 4 entities + 4 repositories (Domain interfaces + Doctrine impls, bound).
  TaskMailLink.task now references TaskInterface (contract) instead of the
  concrete PM Task. Link/unlink/list-mails controllers load tasks via
  TaskRepositoryInterface; MailCreateTaskController keeps the concrete Task
  (instantiation) — documented Mail->PM coupling.
- Domain (MailProviderInterface, exception), Application (5 DTOs, MailSyncService,
  MailSyncRequested message + handler), Infrastructure (ImapMailProvider +
  MimeHeaderDecoder, MailAccessChecker, 2 console commands, 12 controllers,
  ApiPlatform state + MailSettings resource). TokenEncryptor stays shared.
- doctrine mapping Mail; messenger routing repointed; services.yaml repo +
  provider bindings; MailModule registered (id mail, mail.access/configure).
- #[Auditable] + Timestampable on MailConfiguration only (additive migration);
  IMAP data entities keep their own sync timestamps.

163 tests green, mapping valid, no route regression, cs-fixer clean.

src/Module/Mail/Infrastructure/Doctrine/DoctrineMailConfigurationRepository.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Module\Mail\Infrastructure\Doctrine;
+
+use App\Module\Mail\Domain\Entity\MailConfiguration;
+use App\Module\Mail\Domain\Repository\MailConfigurationRepositoryInterface;
+use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
+use Doctrine\Persistence\ManagerRegistry;
+
+/**
+ * @extends ServiceEntityRepository<MailConfiguration>
+ */
+class DoctrineMailConfigurationRepository extends ServiceEntityRepository implements MailConfigurationRepositoryInterface
+{
+    public function __construct(ManagerRegistry $registry)
+    {
+        parent::__construct($registry, MailConfiguration::class);
+    }
+
+    public function findSingleton(): ?MailConfiguration
+    {
+        return $this->createQueryBuilder('m')
+            ->setMaxResults(1)
+            ->getQuery()
+            ->getOneOrNullResult()
+        ;
+    }
+}