Waitlist handling

When a slot reaches capacity and the system throws an unhandled database constraint violation or returns an HTTP 500, the customer receives no actionable information: they cannot retry a different slot, join a waitlist, or understand what happened. An unhandled capacity error produces the same symptom as a server crash from the user's perspective. Even without a formal waitlist feature, returning a structured 409 with a descriptive message gives the frontend enough information to guide the user — a minimum viable response to a predictable state the system already knows about.

Handle the capacity-exceeded case explicitly in your booking handler (e.g., src/app/api/bookings/route.ts). Return a 409 with a structured body so the frontend can render a useful message.

if (currentCount >= slot.maxCapacity) {
  // If waitlist is supported:
  if (req.body.joinWaitlist) {
    await db.waitlistEntry.create({ data: { slotId, userId } });
    return res.status(202).json({ status: 'waitlisted' });
  }
  return res.status(409).json({
    code: 'SLOT_FULL',
    message: 'This slot is fully booked.',
  });
}