All travelynx and IRIS timestamps are Europe/Berlin. And the DateTime objects
know that they are Europe/Berlin. However, when inserting into a timestamp with
time zone column, Mojo::Pg or one of the backend DBI/DBD libraries does not
respect the time zone. So, UTC+1 or UTC+2 timestamps are treated as UTC+0,
resulting in major mayhem.

(yes, there are actually spambots flooding people with unsolicited
registration e-mails)

This ensures unambiguous checkins for ring lines. See
https://github.com/Traewelling/traewelling/commit/eaf975a3337a151c4759428747342ce49eadea47

TODO: handle CET -> CEST transition (there's a day each year where 2am does not
exist and DateTime does not like it)

HTTP 406 indicates that a user has not accepted the ToS / privacy policy

Issue has been fixed