A failed Stalwart calendar write during confirmation no longer deletes the
booking + SlotLock. The booking stays 'pending' with its lock retained, and a
new @Cron worker (every 2 min, max 5 attempts by default) re-drives the write:
on success it promotes to 'confirmed' and sends the confirmation email; after
the cap it moves to the terminal 'calendar_failed' state and releases the lock.
Tracks calendarWriteAttempts + lastCalendarError on the Booking. The public
confirm endpoint still throws 503 on a failed first write (preserving the DoD:
never surface a confirmed booking without a calendar event); the pending row is
left for the background retry to finish.
Ronni Baslund