Refuse an image URL host that no URL host can be

Only @ was refused in the host of a /v1/image/ URL, so a ? written as
%3F gave the host cdn.example.com?x=1, which Go's URL parser reads as
cdn.example.com with a query: it passed the allowlist and each text
after the ? was cached under keys of its own. The host must now be a
host name or an IPv4 address (letters, digits, -, _ and .) or an IPv6
address in brackets, either one optionally followed by : and a port;
anything else is refused with 400. An IPv6 address without brackets,
which needed a signature, is refused too.

Model: opus-5-5

README.md

@@ -575,7 +575,11 @@ pixa lower-cases the host of a `/v1/image/` URL before anything else uses it, so
 `CDN.Example.com` is matched, fetched, cached and counted against
 `upstream_connections_per_host` as `cdn.example.com`. Such a URL whose host has
 a port, as in `cdn.example.com:8443`, or a `:` with no port, needs a signature
-even when the host is allowlisted. A host holding `@` is refused with 400.
+even when the host is allowlisted. The host must be a host name or an IPv4
+address (letters, digits, hyphens, underscores and dots) or an IPv6 address in
+brackets, either one optionally followed by `:` and a port. Any other host, such
+as one holding `@`, or a `?`, `#`, `%`, `\` or space written percent-encoded, is
+refused with 400.
 
 ### Configuration