Dp/develop2
FL-1110 is not yet shipped and no consumer depends on this event, so the contract is changed outright (no deprecation/compat shim):
- remove ExternalClientID (D-48: personal data, dereferenceable to PII via the SevenRooms API key + unstable G-2)
- add CustomerID *uuid.UUID (omitempty): FL-customer link set by ETL forward-attribution (phone match) or AMS history-pull; nil for non-FL guests; customerId is NOT PII
- CI guard test bans 'clientid' (plus PII names) from the struct NOTE: cascades to ETL (reservation_sink) and AMS (analytics) which must drop their external_client_id usage — coordinated change, workspace won't build until done.