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/image_errors_internal_test.go

@@ -467,7 +467,7 @@ func TestImageRoutes_SameErrorAnswers(t *testing.T) {
 		{"no free upstream connection", httpfetcher.ErrTooManyConnections,
 			http.StatusServiceUnavailable, slog.LevelWarn},
 		{"Content-Type not a source format",
-			fmt.Errorf("%w: image/svg+xml", httpfetcher.ErrInvalidContentType),
+			fmt.Errorf("%w: image/tiff", httpfetcher.ErrInvalidContentType),
 			http.StatusUnsupportedMediaType, slog.LevelWarn},
 		{"any other failure", io.ErrUnexpectedEOF,
 			http.StatusInternalServerError, slog.LevelError},