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

With cdn.example.com allowlisted, CDN.Example.com and
cdn.example.com:8443 passed the allowlist too, and each spelling had its
own cache entries and its own connections under
upstream_connections_per_host. The image URL parser now lower-cases the
host and refuses one holding "@" with ErrInvalidHost (400).
ValidateRequest counts a host followed by ":", with or without a port,
as not allowlisted, so it needs a signature. That check is not in
allowlist.IsAllowed, which also matches referer_blocklist. A URL is now
signed over the lower-cased host; README.md says so.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -144,6 +144,16 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 		return nil, ErrMissingHost
 	}
 
+	// A URL reads what comes before "@" as a user name and password, and the
+	// rest as the host
+	if strings.Contains(host, "@") {
+		return nil, fmt.Errorf("%w: %q holds @", ErrInvalidHost, host)
+	}
+
+	// Host names do not depend on letter case, so the allowlist, the upstream
+	// connection limits and the cache see one spelling of each host
+	host = strings.ToLower(host)
+
 	// Extract query string if present in path
 	if qIndex := strings.Index(path, "?"); qIndex != -1 {
 		query = path[qIndex+1:]