Add “Non‑refundable booking” indicator + export filter for Airbnb reservations

Hosts need a way to quickly identify which Airbnb reservations were booked using Airbnb’s Non‑refundable option (non‑refundable discount/rate plan). Add a reservation-level field/column in Metrics/Reservations (and in CSV export) that clearly flags non‑refundable bookings, so hosts can filter and report on them without having to cross-check each reservation in Airbnb.