Reserves a slot on a piece of equipment for a specific user. Set bookedForID to the user, start/end to the time range, and bookingType to User.

Role bookings are not accepted on this endpoint — submit those through the role-booking flow instead. The target must be equipment (not a regular storage unit), the booked-for user must belong to the current subgroup, and the caller must be a manager of the equipment or be booking for themselves. The end time must be after the start.