Show a maintenance page instead of a blank 500 and update in one click

Emergency layer logs fatal errors to var/log/notfall.log and answers 503.
The system page checks for a new release on load, shows its notes, runs
backup, download, switch, migration and health check in one flow with live
progress, and switches back automatically when the new version fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

config/services.yaml

@@ -63,6 +63,9 @@ services:
         arguments:
             $secret: '%kernel.secret%'
             $siteOrigin: '%env(SITE_ORIGIN)%'
+    App\EventSubscriber\ErrorLogSubscriber:
+        arguments:
+            $debug: '%kernel.debug%'
     App\Controller\SeoController:
         arguments:
             $siteOrigin: '%env(SITE_ORIGIN)%'