`/api/auth/verify` checked any username and password against the JMAP
server without a limit. Every probe reaches the server from the
webmail's own address, so the route was a password oracle and enough
wrong passwords could trip Stalwart's authentication ban against the
webmail itself, locking out every user.
Wrong passwords now use up a budget of five per client and thirty
across all clients per fifteen minutes; right ones are refunded. Over
budget the route answers `inconclusive` and the browser probes the JMAP
server itself, as it did before the pre-check existed.