- emailIntakeService.roundTripTest(): sends a uniquely-tagged email through the
saved SMTP config to the IMAP mailbox (imap_user), then polls IMAP up to 30s
for that subject token; deletes the test message on arrival so it never hits
the accounting inbox. Returns {ok, seconds, recipient} or a typed reason.
- route POST /admin/email/incoming-config/roundtrip (email.send)
- IMAP card: 'Round-trip test' button beside 'Test connection' + Save; toast
reports recipient + delivery time. Distinct reasons mapped (smtp/imap
unconfigured, send_failed, not_received→504).
- en/de strings