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_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"
 )