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/composer.json

@@ -25,11 +25,13 @@
         "symfony/flex": "^2",
         "symfony/framework-bundle": "7.4.*",
         "symfony/http-client": "7.4.*",
+        "symfony/messenger": "7.4.*",
         "symfony/process": "7.4.*",
         "symfony/property-access": "7.4.*",
         "symfony/property-info": "7.4.*",
         "symfony/rate-limiter": "7.4.*",
         "symfony/runtime": "7.4.*",
+        "symfony/scheduler": "7.4.*",
         "symfony/security-bundle": "7.4.*",
         "symfony/serializer": "7.4.*",
         "symfony/twig-bundle": "7.4.*",
@@ -92,6 +94,9 @@
         "phpstan/extension-installer": "^1.4",
         "phpstan/phpstan": "^2.1",
         "phpstan/phpstan-doctrine": "^2.0",
-        "phpstan/phpstan-symfony": "^2.0"
+        "phpstan/phpstan-symfony": "^2.0",
+        "phpunit/phpunit": "^12.5",
+        "symfony/browser-kit": "7.4.*",
+        "symfony/css-selector": "7.4.*"
     }
 }