Add Stay Room

One room the traveller chose on a stay.

StayRoom is to Stay what Stay is to canonical_places — the traveller's instance of a room, not the room itself. See docs/HOTEL_ROOM_MODEL_ASSESSMENT.md §5.

One StayRoom is exactly one LiteAPI offer, and therefore one prebook and one booking: an offer is one room type priced for N occupancies, so N rooms of the SAME type are N entries in occupancies, never N StayRooms. Two DIFFERENT room types cannot share an offer, so those are two StayRooms and two bookings.

Identity is hotel_id + mapped_room_id and nothing else. Everything else — how many travellers and their ages, board, refundability, the price — is a choice about that room and may change without making it a different room.