Implementiere echtes sendmail für den Server. Lokal gibts kein mail und keine outbox mehr

development-system/tests/booking-backend-config.test.php

@@ -11,6 +11,11 @@ function expectSame(string $expected, string $actual, string $message): void
     }
 }
 
+function expectContains(string $needle, string $haystack, string $message): void
+{
+    if (!str_contains($haystack, $needle)) throw new RuntimeException($message);
+}
+
 $configFile = tempnam(sys_get_temp_dir(), 'sufw-booking-config-');
 if ($configFile === false) throw new RuntimeException('Temporary configuration file could not be created.');
 
@@ -27,6 +32,18 @@ try {
         bookingDatabaseUrl('mysql://environment:secret@database:3306/booking', $configFile),
         'DATABASE_URL must take precedence over the configuration file.'
     );
+    expectSame('disabled', bookingMailTransport('disabled'), 'Local mail delivery must be explicitly disabled.');
+
+    $calendar = bookingIcsCalendar([
+        'id' => '44c5d0f9-6c98-4b04-8d25-c32834d21a61',
+        'project' => 'perspektivwerk',
+        'title' => 'Erstgespräch PerspektivWerk',
+        'starts_at' => '2026-09-14 09:00:00',
+        'ends_at' => '2026-09-14 10:00:00',
+        'format' => 'vor_ort',
+    ]);
+    expectContains('BEGIN:VCALENDAR', $calendar, 'The confirmation attachment must be an iCalendar document.');
+    expectContains('UID:44c5d0f9-6c98-4b04-8d25-c32834d21a61@sufw-booking', $calendar, 'The iCalendar attachment must contain the event UID.');
 } finally {
     unlink($configFile);
 }