Modify a booking (date change or special requests)

Runs booking.modify; owner-only — a booking the bearer session does not own answers 404, exactly like a booking that does not exist. The signed-in owner may modify from the booking screen without verificationCode, matching the web booking page. verificationCode is the second half of the OTP flow the modifyBooking chat tool starts: when sent it must be the emailed 6-digit code (single-use, 10-minute expiry), and a wrong or expired code is a 401 with nothing changed. At least one of checkIn, checkOut, specialRequests must be sent (the procedure rejects an empty change with 400). A date change on a supplier-linked booking is a real supplier cancel-and-rebook: expect 409 when the new dates have no availability or the supplier can't amend in place, and 502 when the rebook failed after the original was released. Show the 409/502 message verbatim — it states what happened to the reservation and what to do next.