POST / had no limit, so the signing key could be guessed at no cost. It
is now limited to 5 attempts per minute per client by a new RateLimit
middleware on github.com/go-chi/httprate; an attempt over the limit gets
429 with Retry-After. It counts by the address the ClientIP middleware
resolved through trusted_proxies (an IPv4-mapped address as its IPv4
address, IPv6 by its /64) and runs after the body-size and CSRF checks,
so every attempt that reaches the key comparison is counted. README says
that with the default trusted_proxies a client with a private address
can choose its counted address, and how to close that.
Model: opus-5-5
