is_available only checked whether a slot was booked, so on today's date
slots whose start time had already passed (e.g. 10:00 when it is 12:00)
still showed as bookable until the POST /appointment 422. Treat a slot
with start < now as unavailable in getAllSlotsWithAvailability, and drop
past slots in filterBookedSlots so getAvailableSlots agrees. Future days
are unaffected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
