POST /v1/bookings/{id}/extras

Add a SINGLE extra (delta-add). Body: { extra_id: "ex_X", quantity }. Quantity defaults to 1. If the same extras_id is already on the booking, the existing line's quantity accumulates rather than a duplicate row being inserted (matches native portal behaviour).

Refuses (422) if the extra is already in the booking's package as an included feature — customer would otherwise be double-charged.