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/avif_too_large_internal_test.go

@@ -9,7 +9,7 @@ import (
 	"testing"
 
 	"sneak.berlin/go/pixa/internal/imageprocessor"
-	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // TestAVIFOutputTooLargeAnswers400 checks that respondImageError and
@@ -26,7 +26,7 @@ func TestAVIFOutputTooLargeAnswers400(t *testing.T) {
 	wantBody := fmt.Sprintf("above %d pixels", imageprocessor.MaxAVIFOutputPixels)
 
 	plain := httptest.NewRecorder()
-	h.respondImageError(plain, &imgcache.ImageRequest{}, err)
+	h.respondImageError(plain, &imagerequest.Request{}, err)
 
 	encrypted := httptest.NewRecorder()
 	h.handleImageError(encrypted, err)