Fetch an IPv6 source host written without a port (closes #295)

The fetcher's check of a source URL cut a port off the host with
net.SplitHostPort, which fails when an IPv6 address in brackets has no
port, so the brackets stayed on, the name lookup failed and the request
answered 500. The check now parses the URL and takes the host from
Hostname(), which drops the brackets with or without a port, so the
address is looked up and a private one is refused with 403.

Model: opus-5-5

internal/httpfetcher/httpfetcher.go

@@ -473,16 +473,16 @@ func validateURL(ctx context.Context, rawURL string, allowHTTP bool) error {
 		return ErrUnsupportedScheme
 	}
 
-	// Parse to extract host
-	host := extractHost(rawURL)
-	if host == "" {
-		return ErrInvalidHost
+	parsedURL, err := neturl.Parse(rawURL)
+	if err != nil {
+		return fmt.Errorf("%w: %w", ErrInvalidHost, err)
 	}
 
-	// Remove port if present
-	h, _, err := net.SplitHostPort(host)
-	if err == nil {
-		host = h
+	// Hostname drops any port, and the brackets of an IPv6 address whether
+	// or not a port follows them
+	host := parsedURL.Hostname()
+	if host == "" {
+		return ErrInvalidHost
 	}
 
 	// Block obvious localhost patterns