Send a test mail from the admin and log why delivery failed

Form submissions marked as not sent gave no reason: the transport
exception was swallowed. Every send now goes through MailCheck, which
logs the server's answer and a hint in German to var/log/mail.log
(server lines only, no credentials). The system screen shows the mail
route, sends a test mail to any address and lists recent attempts
(ADR 121).

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

config/packages/test/mailer.yaml

@@ -2,3 +2,8 @@ framework:
     mailer:
         # Tests never send real mail; the transport collects messages in memory instead.
         dsn: 'null://null'
+
+parameters:
+    # Das Versandprotokoll der Tests gehört nicht nach var/log der echten Installation.
+    kant.mail_log: '%kernel.cache_dir%/mail.log'
+    kant.mailer_dsn: 'null://null'

config/services.yaml

@@ -10,6 +10,8 @@ parameters:
     # das Projekt schreiben, und eine Installation mit schreibgeschuetztem public/ ebenfalls.
     kant.fonts_dir: '%kernel.project_dir%/public/assets/fonts'
     kant.asset_dir: '%kernel.project_dir%/public/assets'
+    kant.mail_log: '%kernel.project_dir%/var/log/mail.log'
+    kant.mailer_dsn: 'sendmail://default'
 
 services:
     _defaults:
@@ -59,6 +61,12 @@ services:
     App\Content\StyleBundle:
         arguments:
             $assetDirectory: '%env(default:kant.asset_dir:ASSET_DIRECTORY)%'
+    # Versand und Protokoll (ADR 121). MAIL_LOG nur fuer Tests, damit sie nicht ins echte var/log
+    # schreiben.
+    App\Ops\MailCheck:
+        arguments:
+            $dsn: '%env(default:kant.mailer_dsn:MAILER_DSN)%'
+            $logFile: '%env(default:kant.mail_log:MAIL_LOG)%'
     App\Controller\FormController:
         arguments:
             $secret: '%kernel.secret%'