Adds clientIP() (reads the rightmost X-Forwarded-For hop via
Header.Values, since Traefik appends the peer address it actually
observed and the leftmost entries are client-controlled) and
loginLimiter, an in-memory per-IP counter that blocks after
loginMaxFailures within loginWindow. No routes wire these up yet —
that lands in Task 5.
sulthan
6030a985fa