Permanently delete an archived test case

Permanently deletes a test case that has already been archived. Archive it first with POST /api/cases/{caseId}/archive; a case that is not archived is refused with 409. The case is removed from any plans it belongs to, and its rows in past plan runs are removed with it, so those runs show fewer cases than their stored totals. The agent sessions themselves are kept but detached from the case. This cannot be undone.

Requires an organization admin or owner. Member-scoped API keys receive 403; this permission was tightened and previously accepted any member.