Include quality and fit in the URL signature (closes #60)

The signed data is now
host:path:query:width:height:format:expiration:quality:fit. The route
turns a missing q into 85 and a missing fit into cover before checking
the signature, so those are the values signed for a URL without them;
imgcache fills both from the parsed request.

imgcache.Service.GenerateSignedURL now writes q and fit into the URL
next to sig and exp, first setting an unset quality or fit to 85 or
cover, so a generated URL verifies for the values it signed.

The known-answer vectors in golden_test.go, including one for quality
40 and fit contain, and the README signature section describe the new
format.

Model: opus-4-8 (implementation); opus-5-5 (rework)

internal/signature/signature.go

@@ -37,6 +37,15 @@ type Request struct {
 	Height int
 	// Format is the requested output format (e.g. "webp").
 	Format string
+	// Quality is the requested output quality (1-100) for lossy formats.
+	// It is the effective value the request resolves to: callers pass the
+	// default quality when the request omits the parameter, so an omitted
+	// quality signs identically to that same value stated explicitly.
+	Quality int
+	// FitMode is how the image is fit into the requested dimensions
+	// (e.g. "cover"). Like Quality it is the effective value: callers pass
+	// the default fit mode when the request omits the parameter.
+	FitMode string
 	// Signature is the HMAC signature to verify.
 	Signature string
 	// Expires is the signature expiration timestamp.
@@ -56,7 +65,8 @@ func New(secretKey string) *Signer {
 }
 
 // Sign generates an HMAC-SHA256 signature for the given request.
-// The signature covers: host + path + query + width + height + format + expiration.
+// The signature covers: host + path + query + width + height + format +
+// expiration + quality + fit.
 func (s *Signer) Sign(req *Request) string {
 	data := s.buildSignatureData(req)
 	mac := hmac.New(sha256.New, s.secretKey)
@@ -68,7 +78,8 @@ func (s *Signer) Sign(req *Request) string {
 
 // Verify checks if the signature on the request is valid and not expired.
 // Signatures are exact-match only: every component of the signed data
-// (host, path, query, dimensions, format, expiration) must match exactly.
+// (host, path, query, dimensions, format, expiration, quality, fit) must
+// match exactly.
 // No suffix matching, wildcard matching, or partial matching is supported.
 // A signature for "cdn.example.com" will NOT verify for "example.com" or
 // "other.cdn.example.com", and vice versa.
@@ -142,11 +153,13 @@ func (s *Signer) GenerateSignedURL(
 }
 
 // buildSignatureData creates the string to be signed.
-// Format: "host:path:query:width:height:format:expiration"
+// Format: "host:path:query:width:height:format:expiration:quality:fit"
 // All components are used verbatim (exact match). No normalization,
-// suffix matching, or wildcard expansion is performed.
+// suffix matching, or wildcard expansion is performed. Quality and fit
+// are the effective transform values, so replaying a signed URL with a
+// different quality or fit mode fails verification.
 func (s *Signer) buildSignatureData(req *Request) string {
-	return fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d",
+	return fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%d:%s",
 		req.SourceHost,
 		req.SourcePath,
 		req.SourceQuery,
@@ -154,6 +167,8 @@ func (s *Signer) buildSignatureData(req *Request) string {
 		req.Height,
 		req.Format,
 		req.Expires.Unix(),
+		req.Quality,
+		req.FitMode,
 	)
 }