Newsletter: Address to self.

An empty To: header is rejected by WEB.DE.  Therefore, adding a
primary recipient is necessary even if no obvious one exists.

This change makes the NewsletterSender's own address the primary
recipient, circumventing the problem.

Change-Id: I78febdd1f57b3f5737d83334972867ec9e4781d8
diff --git a/src/main/java/eu/mulk/mulkcms2/benki/newsletter/NewsletterSender.java b/src/main/java/eu/mulk/mulkcms2/benki/newsletter/NewsletterSender.java
index 7a52d32..8c8c9b6 100644
--- a/src/main/java/eu/mulk/mulkcms2/benki/newsletter/NewsletterSender.java
+++ b/src/main/java/eu/mulk/mulkcms2/benki/newsletter/NewsletterSender.java
@@ -37,6 +37,9 @@
   @ConfigProperty(name = "mulkcms.newsletter.time-zone")
   ZoneId newsletterTimeZone;
 
+  @ConfigProperty(name = "quarkus.mailer.from")
+  String senderAddress;
+
   @PersistenceContext EntityManager em;
 
   @CheckedTemplate
@@ -100,6 +103,7 @@
     var sendJob =
         mailText
             .subject(String.format("MulkCMS newsletter #%d", newsletterNumber))
+            .to(senderAddress)
             .bcc(subscriberEmails)
             .send();
     sendJob.toCompletableFuture().get(10000, TimeUnit.SECONDS);