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 CBOR field names, so tokens made before still decode. 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/svg_as_avif_internal_test.go

@@ -8,7 +8,7 @@ import (
 	"testing"
 
 	"sneak.berlin/go/pixa/internal/encurl"
-	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // svgServedAsAVIF is an SVG whose first bytes pass the first-bytes check for
@@ -38,10 +38,13 @@ func TestImageProxyRefusesSVGServedAsAVIF(t *testing.T) {
 		t.Fatalf("encurl.NewGenerator: %v", err)
 	}
 
-	encryptedURL := func(format imgcache.ImageFormat) string {
-		token, err := generator.Generate(&encurl.Payload{
-			SourceHost: upstreamHost, SourcePath: "/drawing.avif",
-			Width: 32, Height: 24, Format: format,
+	encryptedURL := func(format imagerequest.Format) string {
+		token, err := generator.Generate(&imagerequest.Request{
+			Source: imagerequest.Source{Host: upstreamHost, Path: "/drawing.avif"},
+			Transform: imagerequest.Transform{
+				Size:   imagerequest.Size{Width: 32, Height: 24},
+				Format: format,
+			},
 		})
 		if err != nil {
 			t.Fatalf("encurl Generate: %v", err)
@@ -53,8 +56,8 @@ func TestImageProxyRefusesSVGServedAsAVIF(t *testing.T) {
 	for _, target := range []string{
 		"/v1/image/" + upstreamHost + "/drawing.avif/32x24.png",
 		"/v1/image/" + upstreamHost + "/drawing.avif/64x48.orig",
-		encryptedURL(imgcache.FormatPNG),
-		encryptedURL(imgcache.FormatOriginal),
+		encryptedURL(imagerequest.FormatPNG),
+		encryptedURL(imagerequest.FormatOriginal),
 	} {
 		rec := httptest.NewRecorder()
 		s.ServeHTTP(rec, httptest.NewRequestWithContext(