chore: add symfony/messenger + symfony/scheduler + phpunit infrastructure

- composer.json : symfony/messenger, symfony/scheduler, symfony/test-pack
- PHPStan fix : is_string() pour narrower mixed dans buildForecastGeometry
- phpunit.dist.xml + tests/bootstrap.php + bin/phpunit pour la suite de tests
- Suppression de src/Schedule.php (schedule vide généré par Flex, inutile)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env

@@ -55,3 +55,10 @@ VAPID_PUBLIC_KEY=
 VAPID_PRIVATE_KEY=
 APP_PUBLIC_URL=https://radarsargasses971.com
 ###< web-push ###
+
+###> symfony/messenger ###
+# Choose one of the transports below
+# MESSENGER_TRANSPORT_DSN=amqp://guest:guest@localhost:5672/%2f/messages
+# MESSENGER_TRANSPORT_DSN=redis://localhost:6379/messages
+MESSENGER_TRANSPORT_DSN=doctrine://default?auto_setup=0
+###< symfony/messenger ###