Change belief horizon column type to int to save space

[nhoening]

The belief horizon column takes 16 bytes, but could also store minutes/seconds as integer, which would reduce 12 bytes per belief, which certainly a fifth of all our data size.

We need to find out how to migrate this (in the code/ORM data model, which should lead to an alembic migration).

Then, we need to know how timely-beliefs can translate this from and into a timedelta:

• in input, we need to translate timedelta to int when a BeliefsDataFramw is created and in querying - query logic should still work the same, we only need to translate timedeltas to integer if a belief horizon parameter is given
• in its output, it would be best to only translate (to timedelta) when someone accesses this column in the BeliefsDataFrame