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/handlers/imageenc_token_internal_test.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // requireServedPhoto requires that rec answers 200 with the JPEG at photoPath
@@ -59,11 +60,14 @@ func TestHandleImageEnc_RejectedToken(t *testing.T) {
 
 	h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
 
-	photo := encurl.Payload{
-		SourceHost: signedHost,
-		SourcePath: photoPath,
-		Width:      50,
-		Height:     50,
+	photo := imagerequest.Request{
+		Source: imagerequest.Source{
+			Host: signedHost,
+			Path: photoPath,
+		},
+		Transform: imagerequest.Transform{
+			Size: imagerequest.Size{Width: 50, Height: 50},
+		},
 	}
 
 	valid, err := h.encGen.Generate(&photo)
@@ -72,7 +76,7 @@ func TestHandleImageEnc_RejectedToken(t *testing.T) {
 	}
 
 	expiredPhoto := photo
-	expiredPhoto.ExpiresAt = time.Now().Add(-time.Minute).Unix()
+	expiredPhoto.Expires = time.Now().Add(-time.Minute)
 
 	expired, err := h.encGen.Generate(&expiredPhoto)
 	if err != nil {