Leave the customer
A contact removing their own access. Not an owner action — every member may do it — but the last-owner guard still applies: leaving must not orphan the team.
Authorizations
A contact session token, uk_ct_…. Belongs to one of the developer's own users, and reads only that user's data.
Headers
The customer this call is acting inside — the same arrangement as X-Organization-Id, one plane down. Switching customers is navigation, not a mutation: nothing is stored, you send a different header, and the membership JOIN behind it decides what you may do there. An id you are not a member of matches no row and answers 404. Absent, the contact's oldest membership answers.
Response
Left.