Terminate an active hiring contract (org side)

Terminate an active hiring. The SDR still receives a final prorated invoice at the next monthly billing run covering work through the termination date.

Auth: requires a personal API key whose user has the Admin or Owner role in the calling org (require_org_admin). Org-scoped (machine) keys are refused — see developer_api.dependencies.admin. The rationale: termination ends a paid commercial relationship and is not reversible from this surface (the client must send a fresh hire offer to re-engage). Restricting the trigger to org admins matches the org's intent that contract lifecycle is a leadership-level decision.