Revoke a child API key
Soft-revokes a child API key. The key cuts over IMMEDIATELY at data-plane endpoints (`/v1/calls`, `/v1/messages`, etc.) — there is no 60-second grace window for child keys. Subsequent requests with the revoked key return `invalid_api_key`. The returned row carries `revoked_at` populated; calling DELETE again returns 404 (idempotent at the user-visible level).
