Support SVG sources (closes #68)

The fetcher accepts image/svg+xml again and internal/magic detects SVG.
A raster format renders the SVG with librsvg at the density that makes
it cover the output size, so an enlarged SVG stays sharp; the rendered
size is held to MaxInputDimension. orig of an SVG at the orig size is
the SVG unchanged, cached as a variant; at any other size it is
ErrSVGNotOriginalSize, answered 400. The cache reads up to 1024 first
bytes of a file to find its format, as an SVG's root element can come
after a prolog. Tests that pinned SVG as refused now expect it to be
read.

Model: opus-5-5

internal/handlers/unsupported_source_internal_test.go

@@ -15,7 +15,7 @@ import (
 
 // TestUnsupportedSourceFormatAnswers415 checks that respondImageError, which
 // answers the errors of both image routes, answers 415 with a clear error when
-// the upstream answers with a content type pixa does not accept, such as SVG,
+// the upstream answers with a content type pixa does not accept, such as TIFF,
 // and when libvips does not read a source as the format its first bytes
 // show, wrapped as the service wraps them.
 func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
@@ -23,11 +23,11 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 
 	h := &Handlers{log: slog.New(slog.DiscardHandler)}
 	req := &imagerequest.Request{
-		Source: imagerequest.Source{Host: "img.example.com", Path: "/a.svg"},
+		Source: imagerequest.Source{Host: "img.example.com", Path: "/a.tiff"},
 	}
 
 	for _, err := range []error{
-		fmt.Errorf("upstream fetch failed: %w: image/svg+xml",
+		fmt.Errorf("upstream fetch failed: %w: image/tiff",
 			httpfetcher.ErrInvalidContentType),
 		fmt.Errorf("image processing failed: %w: libvips does not read the input as avif",
 			imageprocessor.ErrUnsupportedInputFormat),