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/tests/bootstrap.php

@@ -0,0 +1,13 @@
+<?php
+
+use Symfony\Component\Dotenv\Dotenv;
+
+require dirname(__DIR__).'/vendor/autoload.php';
+
+if (method_exists(Dotenv::class, 'bootEnv')) {
+    (new Dotenv())->bootEnv(dirname(__DIR__).'/.env');
+}
+
+if ($_SERVER['APP_DEBUG']) {
+    umask(0000);
+}