Make backup report hostname configurable

etc/app-backup/app-backup.conf

@@ -41,6 +41,9 @@ MAIL_TO="johannes"
 # Empty means app-backup@<FQDN>. Existing sender/subject/tail settings are preserved.
 MAIL_FROM=""
 MAIL_SUBJECT_PREFIX="[app-backup]"
+# Host in subject/report only; empty uses the system FQDN. Sender is unchanged.
+# Example for Gemini: MAIL_HOSTNAME="gemini.jr-it-services.de"
+MAIL_HOSTNAME=""
 MAIL_INCLUDE_LOG_TAIL_LINES=200
 MAIL_SENDMAIL_BIN="/usr/sbin/sendmail"
 MAIL_SEND_TIMEOUT=30