A semester groups a recurring set of classes that customers enrol into as a whole.

The id returned here is the stable identifier of the group. Individual classes report it as semesterId on GET /api/v2/host/sessions, so the two endpoints can be joined to attribute a class - or a sale made against one - to the semester it belongs to.

activeFrom and activeTo filter on overlap, not containment: a semester running January to June is returned for activeFrom=2021-03-01&activeTo=2021-03-31.