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_test.go

@@ -15,6 +15,8 @@ const (
 	testPath       = "/photos/cat.jpg"
 	testFormatWebP = "webp"
 	testFormatPNG  = "png"
+	testFitCover   = "cover"
+	testFitContain = "contain"
 	testSignedPath = "/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp"
 	testSig        = "abc123"
 )