feat(mail) : MailSyncRequested message + handler + messenger.yaml transport async Doctrine

- App\Message\MailSyncRequested (optionnel folderPath)
- App\MessageHandler\MailSyncRequestedHandler delegue a MailSyncService::syncFolder ou syncAll
- messenger.yaml : transport async via Doctrine DSN, retry 3x exponentiel, failure transport
- en test : transport in-memory (sync immediat)
- migration Version20260519220000 : cree messenger_messages table (idempotente, IF NOT EXISTS)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Message/MailSyncRequested.php

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Message;
+
+final readonly class MailSyncRequested
+{
+    public function __construct(
+        public ?string $folderPath = null,
+    ) {}
+}