Cancelled booking sends a CANCEL .ics to remove the event from the customer's calendar

When a booking is cancelled, the customer's calendar app needs a METHOD:CANCEL invite with the original UID to remove the existing event. Without it, the customer's calendar continues to show the appointment — they may still show up. Sending a METHOD:PUBLISH invite on cancellation makes the situation worse: the calendar client treats it as a new event confirmation rather than a removal. RFC 5546 defines METHOD:CANCEL specifically for this case. The failure is silent from the system's perspective (the email is sent, the queue job succeeds) but creates a confused customer experience and potential unnecessary no-shows.

Add a generateCancelICS() function to src/lib/calendar.ts that uses the same stable UID as the original event and sets METHOD:CANCEL and STATUS:CANCELLED. Attach it to the cancellation email:

export function generateCancelICS(booking: Booking): string {
  const dtstamp = new Date().toISOString().replace(/[-:]/g, '').split('.')[0] + 'Z'
  return [
    'BEGIN:VCALENDAR',
    'VERSION:2.0',
    'METHOD:CANCEL',
    'BEGIN:VEVENT',
    `UID:${booking.id}@yourdomain.com`, // Same UID as original
    `DTSTAMP:${dtstamp}`,
    'STATUS:CANCELLED',
    `SUMMARY:CANCELLED: ${booking.service_name}`,
    `DTSTART:${formatICSDate(booking.start_time)}`,
    `DTEND:${formatICSDate(booking.end_time)}`,
    'END:VEVENT',
    'END:VCALENDAR',
  ].join('\r\n')
}

Call this from the cancellation email worker, not from the main cancellation handler, to keep it on the retry path.