Count an IPv4-mapped login client by its IPv4 address (closes #66)

A proxy on a dual-stack listener forwards an IPv4 client as ::ffff:a.b.c.d,
whose /64 is the same for every IPv4 client, so one client's failed logins
refused everyone's. The rate limit key now unmaps the address first.

README.md now says that with the default trusted_proxies a client with a
private address can choose its counted address through X-Forwarded-For, and
that setting trusted_proxies to the proxy's own address closes this.

Model: opus-5-5

TODO.md

@@ -33,11 +33,12 @@ exhaustion
 - 2026-09-28 rate limit the login form (closes #66): `POST /` is limited to 5
   attempts per minute per client address, and an attempt over the limit is
   refused with 429 and a `Retry-After` header; the address is the one
-  `internal/clientip` resolves through `trusted_proxies`, and an IPv6 client is
-  counted by its /64; the limit is a `RateLimit` middleware in
-  `internal/middleware` on `github.com/go-chi/httprate`, which the image routes
-  can reuse; the library keeps counts for the current and the previous minute
-  only; documented in `README.md`.
+  `internal/clientip` resolves through `trusted_proxies`, an IPv6 client is
+  counted by its /64, and an IPv4-mapped address as the IPv4 address it
+  carries; the limit is a `RateLimit` middleware in `internal/middleware` on
+  `github.com/go-chi/httprate`, which the image routes can reuse; the library
+  keeps counts for the current and the previous minute only; documented in
+  `README.md`.
 - 2026-09-28 refuse an unparseable `exp` on `/v1/image/` and log swallowed
   cache errors (closes #72): an `exp` in the URL that is not a whole
   number, an empty `exp=` included, is a 400 naming `exp` and the value,