Revoke Organization Invite

Soft-revokes a pending organization invite by setting its status to revoked. The invite row is preserved for audit purposes. Returns 409 if the invite is in any other state (already accepted, declined, expired, or revoked), since revoking an accepted invite would leave a misleading audit trail while the underlying membership row remains untouched. To remove an already-accepted member, use DELETE /api/v1/organizations/members/{user_id} instead. Requires a global API key.