Rate limit login attempts per client address (closes #66)

POST / had no limit, so the signing key could be guessed at no cost. It
is now limited to LoginAttemptsPerMinute (5) attempts per minute per
client by a new RateLimit middleware on github.com/go-chi/httprate. It
counts by the address the ClientIP middleware resolved through
trusted_proxies, an IPv6 client by its /64, and answers an attempt over
the limit with 429 and Retry-After. It runs after the body-size and CSRF
checks, so every attempt that reaches the key comparison is counted. The
image routes can reuse it. README states the limit; TODO narrows the
per-IP item to the image routes.

Model: opus-5-5

README.md

@@ -100,6 +100,13 @@ than once, is refused with 400.
 - `<format>`: one of `orig`, `png`, `jpeg`, `webp`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
 
+The login form (`POST /`) is limited to 5 attempts per minute per client
+address, counting an IPv6 client by its /64; an attempt over the limit is
+refused with 429 and a `Retry-After` header. Behind a reverse proxy the client
+address comes from `X-Forwarded-For` only when the proxy's address is in
+`trusted_proxies`; otherwise all users behind the proxy are counted as one
+client.
+
 ### Source Hosts
 
 Source hosts may be allowlisted in the configuration. Non-allowlisted