Multi-backend support

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)