fix: handle episode deletions in timeslot repetition_of_id field

The update logic of the repetition_of field did not consider cases in
which the episode field of TimeSlot objects is deleted.

This fix populates the TimeSlot’s repetition_of_id field with a subquery
instead of using a separate database field for it.

The decision to keep repetition_of as a separate database field was made
on the assumption that this would be a performance problem. However, the
subquery for the repetition_of field is quite performant, especially
with a separate database index. In contrast, the update logic for a
materialized repetition_of field is quite complex and it is easy to miss
cases that should have updated the field.

As long as we don’t have an actual use case in which retrieving
timeslots becomes slow because of this subquery, we should refrain from
materializing the field.