Test that the URL signature covers quality and fit (closes #60)

Add Quality and FitMode to signature.Request, not yet part of the
signed data, and tests that a signature for quality 85 and fit cover is
refused when replayed with another quality or fit: one against
Signer.Verify, one through the image route. The route test also sends
the same signature with no q or fit and with q=85&fit=cover, which must
both pass, and sends URLs built by imgcache.Service.GenerateSignedURL.

The replay cases fail until the next commit.

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.