Take the signature, the expiry and the URL scheme off the image request (closes #314)

The image request now holds only its source and its transform, exactly what
the variant key covers. The /v1/image/ handler checks the allowlist and
verifies the signature itself with Signer.Verify(req, sig, expires). Both
image handlers keep the URL's expiry as a local value for Cache-Control. The
image service holds no signer, allowlist or AllowHTTP, and ValidateRequest is
gone. The fetcher takes the source and builds its URL with Source.URL from its
own AllowHTTP, so no URL string is built and parsed again. chooseAutoFormat
returns the format it chooses instead of writing into the request. The signed
URL format and every route's answers are unchanged.

Model: opus-5-5

internal/imagerequest/urlparser.go

@@ -126,15 +126,15 @@ func parseImageComponents(remainder string) (*Request, error) {
 		return nil, ErrMissingHost
 	}
 
-	// Only the forms hostRegex describes are accepted. A "?", "#" or "@" in
-	// the host would make Go's URL parser, which the allowlist check and the
-	// fetcher use, read a different host from the one allowlisted and cached
+	// Later steps put the host unparsed into the allowlist check, the cache key
+	// and the upstream URL, so only the forms hostRegex describes are accepted
 	hostMatch := hostRegex.FindStringSubmatch(host)
 	if hostMatch == nil {
 		return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
 	}
 
-	// Go's URL parser refuses brackets around anything but an IPv6 address
+	// The allowlist check and the fetcher drop the brackets, so brackets
+	// around anything but an IPv6 address would give a host a second spelling
 	if inBrackets := hostMatch[1]; inBrackets != "" {
 		addr, err := netip.ParseAddr(inBrackets)
 		if err != nil || !addr.Is6() {
@@ -147,9 +147,8 @@ func parseImageComponents(remainder string) (*Request, error) {
 	// host
 	host = strings.ToLower(host)
 
-	// Go's URL parser, which the fetcher uses, reads a "#" as the start of a
-	// fragment and drops the rest, so each text after it would be another
-	// spelling, cached under its own keys, of one upstream request
+	// A "#" starts a URL's fragment, so neither the source path nor the source
+	// query may hold one
 	if strings.Contains(path, "#") {
 		return nil, fmt.Errorf(
 			`%w: the source path or query holds a "#"`, ErrInvalidPath)