The trick was to use a helper-field and a (decorated) @post_load method in the model that allows to fill the helper
fields with the values (ints) instead of strings for enums.
Trouble is: We are parsing strings from API/JSON and want to serialize as int (value in IntEnum). The helper
fields also must be skipped when setting up the query. Pretty convoluted, but gets the jon done (finally).
Also extended the database by new field 'interval_end' which is a preparation to allow not only timestamps but also
intervals when specifying times for participants.
