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/handlers/unsupported_source_internal_test.go

@@ -10,7 +10,7 @@ import (
 
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
-	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // TestUnsupportedSourceFormatAnswers415 checks that both image routes answer
@@ -21,7 +21,9 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 	t.Parallel()
 
 	h := &Handlers{log: slog.New(slog.DiscardHandler)}
-	req := &imgcache.ImageRequest{SourceHost: "img.example.com", SourcePath: "/a.svg"}
+	req := &imagerequest.Request{
+		Source: imagerequest.Source{Host: "img.example.com", Path: "/a.svg"},
+	}
 
 	for _, err := range []error{
 		fmt.Errorf("upstream fetch failed: %w: image/svg+xml",