Read a trusted_proxies entry in the IPv4-mapped form as its IPv4 range (closes #303)

The client address is matched against trusted_proxies as the IPv4 address
an IPv4-mapped address holds, so an entry such as ::ffff:10.0.0.0/104
trusted nothing. parseCIDRList, which reads both trusted_proxies and
blocked_networks, now keeps such an entry as the IPv4 range it covers and
refuses one shorter than /96 naming the setting it reads.
parseBlockedNetworks, which did this for blocked_networks alone, is gone.

Model: opus-5-5

README.md

@@ -713,16 +713,22 @@ Key settings in more detail:
   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 one, and an
-  explicit list replaces the default. An invalid CIDR aborts startup. Set this
-  to the address pixa sees for requests that come through your proxy, such as
-  `172.17.0.1/32`, when the defaults do not cover it, or to trust nothing else
-  (see the login limit and `image_requests_per_minute` under Routes). For a
-  proxy on the Docker host that connects to pixa over `127.0.0.1`, that address
-  is the gateway of the container's Docker network (`172.17.0.1` on the default
-  bridge), not the proxy's own address; a proxy that connects through another of
-  the host's addresses is seen with that address. To be sure which address it
-  is, set this to `[]` (or `PIXA_TRUSTED_PROXIES` to empty), send a request
-  through the proxy, and read `remoteIP` in pixa's request log line for it
+  explicit list replaces the default. An invalid CIDR aborts startup. An IPv6
+  address in the IPv4-mapped form (`::ffff:a.b.c.d`) is matched only as the IPv4
+  address it holds, so an IPv6 range such as `::/0` trusts neither it nor a
+  plain IPv4 address. An entry written in that form is read as the IPv4 range it
+  covers, so `::ffff:10.0.0.0/104` trusts what `10.0.0.0/8` trusts; such an
+  entry shorter than `/96` covers more than IPv4 addresses and aborts startup.
+  Set this to the address pixa sees for requests that come through your proxy,
+  such as `172.17.0.1/32`, when the defaults do not cover it, or to trust
+  nothing else (see the login limit and `image_requests_per_minute` under
+  Routes). For a proxy on the Docker host that connects to pixa over
+  `127.0.0.1`, that address is the gateway of the container's Docker network
+  (`172.17.0.1` on the default bridge), not the proxy's own address; a proxy
+  that connects through another of the host's addresses is seen with that
+  address. To be sure which address it is, set this to `[]` (or
+  `PIXA_TRUSTED_PROXIES` to empty), send a request through the proxy, and read
+  `remoteIP` in pixa's request log line for it
 - `upstream_fetch_timeout` — time allowed for one fetch from an upstream host,
   as a duration such as `30s` (the default) or `2m`
 - `upstream_max_response_size` — largest upstream response accepted, in bytes;