Lower-case the image URL's host and require a signature for a port (closes #246)

With `cdn.example.com` allowlisted, other letter cases and a host with a port also passed the allowlist; each spelling was fetched and cached on its own with its own per-host connection limit, and a port let an unsigned URL reach any port of the host.

The image URL parser now lower-cases the host and refuses one holding `@` with 400. A host followed by `:`, with or without a port (read as Go's URL parser reads it, unbracketed IPv6 included), never counts as allowlisted, so it needs a signature. The host is signed lower-cased, as the URL writes it, so a signature made over an upper-case spelling is refused with 401.

Model: opus-5-5

internal/imgcache/service.go

@@ -9,6 +9,7 @@ import (
 	"log/slog"
 	"net/url"
 	"runtime/debug"
+	"strings"
 	"time"
 
 	"github.com/dustin/go-humanize"
@@ -218,7 +219,16 @@ func (s *Service) ValidateRequest(req *ImageRequest) error {
 		return fmt.Errorf("invalid source URL: %w", err)
 	}
 
-	if s.allowlist.IsAllowed(parsedURL) {
+	// An allowlist entry names no port, so a host followed by ":", with or
+	// without a port, needs a signature. The port is read from the parsed
+	// URL, as the fetcher reads it: in an IPv6 address written without
+	// brackets, such as 2001:db8::1:8443, it is what follows the last ":".
+	// This is checked here and not in allowlist.IsAllowed, which also matches
+	// referer_blocklist, where a Referer must be blocked whatever its port.
+	hostHasPort := parsedURL.Port() != "" ||
+		strings.HasSuffix(parsedURL.Host, ":")
+
+	if !hostHasPort && s.allowlist.IsAllowed(parsedURL) {
 		return nil
 	}