chore(deps): re-add dragonmantank/cron-expression for the scheduler

This dep was silently dropped by the earlier `composer require
symfony/mercure-bundle --no-scripts` on the prod host (it ran in
no-dev mode and removed packages not currently referenced in
require/require-dev), which crash-looped the worker when
Symfony\Component\Scheduler\Trigger\CronExpressionTrigger tried to
parse the hourly RunImageCleanupMessage cron.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composer.json

@@ -10,6 +10,7 @@
         "doctrine/doctrine-bundle": "^3.2",
         "doctrine/doctrine-migrations-bundle": "^4.0",
         "doctrine/orm": "^3.6",
+        "dragonmantank/cron-expression": "^3.6",
         "phpdocumentor/reflection-docblock": "^6.0",
         "phpstan/phpdoc-parser": "^2.3",
         "symfony/asset": "7.4.*",