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>

config/packages/messenger.yaml

@@ -1,6 +1,28 @@
 framework:
     messenger:
+        failure_transport: failed
+
         transports:
             sync: 'sync://'
 
-        routing: {}
+            async:
+                dsn: '%env(MESSENGER_TRANSPORT_DSN)%'
+                options:
+                    queue_name: default
+                retry_strategy:
+                    max_retries: 3
+                    delay: 1000
+                    multiplier: 2
+                    max_delay: 0
+
+            failed: 'doctrine://default?queue_name=failed&auto_setup=0'
+
+        routing:
+            'App\Message\MailSyncRequested': async
+
+when@test:
+    framework:
+        messenger:
+            transports:
+                async: 'in-memory://'
+                failed: 'in-memory://'