Whether it's for the store as a whole being operated by 1 Member or for each Member and their Services, the Buffer Times need to apply across all bookings.

If we book a Service that requires 15 minutes after for prep, cleanup, or a stretch, it doesn't just apply to someone booking that same Service-- it applies to any Service that might come in immediately after.

Also, the remaining timeslots should reflect starting at those times now. If the 1st Service was 60 minutes, 1pm-2pm with 15 minutes after, then the next bookable time slot should now be 2:15pm-3:15pm across all Services (with the End Times matching the duration based on the adjusted Start Times).