Refuse SVG at the fetcher and never save an unknown input as JPEG (part of #68)

The fetcher no longer accepts image/svg+xml, so an SVG source gets the
refusal every unaccepted upstream type gets, which both image routes now
answer with 415 where they answered 500. orig of an input the processor
cannot name is ErrUnsupportedInputFormat, also answered 415, where it
was saved as JPEG. README.md drops SVG from the source formats and lists
the 415.

Model: opus-5-5

internal/handlers/unsupported_source_internal_test.go

@@ -36,7 +36,7 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 		h.handleImageError(encrypted, err)
 
 		for route, rec := range map[string]*httptest.ResponseRecorder{
-			"/v1/image/": plain, "/v1/e/": encrypted,
+			"HandleImage": plain, "HandleImageEnc": encrypted,
 		} {
 			t.Logf("%s for %v: %d %s", route, err, rec.Code, rec.Body.String())
 

internal/server/svg_internal_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 // TestImageProxyRefusesSVG requests an image the upstream answers as SVG,
-// at its own size and format and resized as PNG, through pixa's router,
+// in its own format (orig) and resized as PNG, through pixa's router,
 // upstream fetcher and image processor. pixa does not accept SVG as a
 // source, so each request is answered 415, never with an image.
 func TestImageProxyRefusesSVG(t *testing.T) {
@@ -24,7 +24,7 @@ func TestImageProxyRefusesSVG(t *testing.T) {
 
 	s, _, _ := startImageProxy(t, upstream)
 
-	for _, sizeFormat := range []string{"orig.orig", "32x24.png"} {
+	for _, sizeFormat := range []string{"64x48.orig", "32x24.png"} {
 		target := "/v1/image/" + upstreamHost + "/drawing.svg/" + sizeFormat
 
 		rec := httptest.NewRecorder()