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.

config/modules.php

@@ -10,6 +10,7 @@ declare(strict_types=1);
 use App\Module\Absence\AbsenceModule;
 use App\Module\Core\CoreModule;
 use App\Module\Directory\DirectoryModule;
+use App\Module\Mail\MailModule;
 use App\Module\ProjectManagement\ProjectManagementModule;
 use App\Module\TimeTracking\TimeTrackingModule;
 
@@ -19,4 +20,5 @@ return [
     ProjectManagementModule::class,
     AbsenceModule::class,
     DirectoryModule::class,
+    MailModule::class,
 ];