Assign an owner to an ownerless connector

Gives a connector that has no owner an owner, and nothing else — every grant is left as it was. Never moves an existing owner: a connector that already has one answers 409 connector_has_owner (that is POST /connector/{slug}/transfer, owner only), and so does a claim that loses a race with another claim or the creator backfill. Requires connector:manage. The target must be an active member of the organization. Naming yourself additionally requires a real edit grant of your own (can_claim) — a bare connector:manage never makes its holder the owner of a connector nobody shared with them (403); naming someone else needs no grant of yours, so a connector:manage holder who reaches nothing may assign it. 404 only for a slug that is not an org-owned connector. Recorded in the audit log as connector.owner_assigned. The 200 is the connector as the CALLER now sees it.