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/.gitignore

@@ -8,3 +8,8 @@
 /var/
 /vendor/
 ###< symfony/framework-bundle ###
+
+###> phpunit/phpunit ###
+/phpunit.xml
+/.phpunit.cache/
+###< phpunit/phpunit ###