Return the seat map for a locked flight fare, per trip and leg.

fareId is the Mercury UUID from POST /v1/flights/fares/build; Mercury reverse-maps it to the upstream fare id server-side.

Passengers identify who to price seats for. passengerId is the stringified 0-based traveller index ("0", "1", …) — never a stored companion or profile id. Omit it for a single traveller; prices are keyed by "0". Free seats have amount: 0.

A leg with no map returns error set and cabin null — this is not a transport failure for the whole response.

A guest-session caller may fetch the map for its own locked fare. A fare locked by another principal does not resolve, so it returns the same 410 an expired fare does.