2783b060

By: Michael Lynch <git@mtlynch.io>

Use generic rate limit error in login handler

Change Limiter.Allow() to return nil or ErrLimitExceeded instead of
bool so callers propagate the error directly. Update the shared-secret
authenticator and login handler to use ratelimit.ErrLimitExceeded,
removing the handler's import of the auth implementation package.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>