unique_calendar_event_per_amends_and_datetimes constraint causes issues

e.g., it does not allow a lesson event to be amended with a documentation & a substitution lesson with, respectively, the same start/end date(time)s

added part::backend type::bug labels

The situation you described should never occur! Amendments shouldn't have multiple branches. If a substitution is added and there is already an amendment, the existing amendment has to be changed to amending the substitution.

e. g. https://edugit.org/AlekSIS/official/AlekSIS-App-Untis/-/blob/master/aleksis/apps/untis/util/mysql/util.py?ref_type=heads#L222