One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's short CBOR field names. VariantKey moves with the function that
makes it. Tests follow the moved and renamed code; the tests of the
removed conversions go with them.

Model: opus-5-5

internal/server/source_url_internal_test.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 	"sneak.berlin/go/pixa/internal/imgcache"
 	"sneak.berlin/go/pixa/internal/session"
 	"sneak.berlin/go/pixa/internal/signature"
@@ -513,12 +514,16 @@ func signedImageURL(t *testing.T, sourceHost string) string {
 	t.Helper()
 
 	path, sig, exp := signature.New(testSigningKey).GenerateSignedURL(
-		&signature.Request{
-			SourceHost: sourceHost,
-			SourcePath: ipv6SourcePath,
-			Format:     string(imgcache.FormatPNG),
-			Quality:    85,
-			FitMode:    string(imgcache.FitCover),
+		&imagerequest.Request{
+			Source: imagerequest.Source{
+				Host: sourceHost,
+				Path: ipv6SourcePath,
+			},
+			Transform: imagerequest.Transform{
+				Format:  imagerequest.FormatPNG,
+				Quality: 85,
+				FitMode: imagerequest.FitCover,
+			},
 		}, time.Hour)
 
 	return path + "?sig=" + sig + "&exp=" + strconv.FormatInt(exp, 10)
@@ -534,9 +539,9 @@ func encryptedImageURL(t *testing.T, sourceHost string) string {
 		t.Fatalf("encurl.NewGenerator: %v", err)
 	}
 
-	token, err := generator.Generate(&encurl.Payload{
-		SourceHost: sourceHost, SourcePath: ipv6SourcePath,
-		Format: imgcache.FormatPNG,
+	token, err := generator.Generate(&imagerequest.Request{
+		Source:    imagerequest.Source{Host: sourceHost, Path: ipv6SourcePath},
+		Transform: imagerequest.Transform{Format: imagerequest.FormatPNG},
 	})
 	if err != nil {
 		t.Fatalf("encurl Generate: %v", err)