Squashed commit of the following:

commit 92518024
Author: Birte Kristina Friesel <birte.friesel@uos.de>
Date:   Fri Jul 26 18:39:46 2024 +0200

    add_or_update station: remove superfluos 'new backend id := old backend id'

commit df21c20c
Author: Birte Kristina Friesel <birte.friesel@uos.de>
Date:   Fri Jul 26 18:35:51 2024 +0200

    revert connection targets min_count to 3

commit be335cef
Author: Birte Kristina Friesel <birte.friesel@uos.de>
Date:   Fri Jul 26 18:20:05 2024 +0200

    mention backend selection in API documentation

commit 9f41828f
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Thu Jul 25 20:19:23 2024 +0200

    use_history: not all backends provide route data in departure monitor

commit 09714b4d
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Thu Jul 25 20:11:44 2024 +0200

    disambiguation: pass correct hafas parameter

commit 8cdf1120
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Thu Jul 25 20:11:28 2024 +0200

    _checked_in: hide Zuglauf link for non-db checkins

commit 7455653f
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Thu Jul 25 20:01:47 2024 +0200

    debug output

commit b9cda07f
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Thu Jul 25 19:09:07 2024 +0200

    fix remaining get_connection_targets / get_connecting_trains_p invocations

commit 2759d725
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Wed Jul 24 20:50:12 2024 +0200

    support non-DB HAFAS backends (WiP)

This speeds up the map significantly and makes coordinates_by_station obsolete

This increases resilience against EVA ID reuse

Closes #144

this avoids a race condition when the worker requests all current checkins and
takes a while before getting to update the data

This is in preparation for HAFAS wagonorder support

this avoid race conditions when a user changes their checkin between the time
of request (old trip id) and the time of data update (new trip id). These
could lead to funny effects such as buses suddenly having S-Bahn carriage
formations.

Decreases severity of #99
Fixes #108

This is a breaking change insofar as that traewelling support is no longer
provided automatically, but must be enabled by providing a traewelling.de
application ID and secret in travelynx.conf. However, as traewelling.de
password login is deprecated and wil soon be disabled, travelynx would break
either way. So we might or might not see travelynx 2.0.0 in the next days.

Automatic token refresh is still todo, but that was the case for password
login as well.

Closes #64

travelynx now always calls checkout with the database-provided arr_eva,
so the edge case in which it provided a station that is not actually on the
route of a train can no longer appear.