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 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

README.md

@@ -100,6 +100,17 @@ 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. With the default `trusted_proxies` (the RFC 1918 ranges), a client
+with a private address can choose the address it is counted by through its own
+`X-Forwarded-For`, whether it connects directly or through the proxy, because
+its own address is trusted too. Setting `trusted_proxies` to the proxy's own
+address closes this.
+
 ### Source Hosts
 
 Source hosts may be allowlisted in the configuration. Non-allowlisted
@@ -205,7 +216,8 @@ Key settings in more detail:
   inside one of these ranges; the logged and login-recorded client
   address is then the rightmost forwarded entry that is not itself a
   trusted proxy. Otherwise the direct peer address is used and the header
-  is ignored, so a client connecting directly cannot spoof its address.
+  is ignored, so a client connecting directly from an address outside
+  these ranges cannot spoof its address.
   An omitted key defaults to the RFC 1918 private ranges (`10.0.0.0/8`,
   `172.16.0.0/12`, `192.168.0.0/16`), since pixa is deployed behind a
   proxy on a private network; an explicitly empty list (`[]`) trusts no