Support SVG sources (closes #68)

The fetcher accepts image/svg+xml again and internal/magic detects SVG.
A raster format renders the SVG at the least size that covers the
output, held to 8192 on a side, through libvips' thumbnail, which
renders at any scale, so an enlarged SVG stays sharp. An SVG's own size
is not limited; one with a side more than 8192 times the other is
ErrSVGTooThin, answered 415. 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. Render time and memory grow with
what the SVG draws; their bound is the owner's decision on #68. Tests that pinned SVG as refused now
expect it to be read.

Model: opus-5-5

internal/imgcache/cache.go

@@ -449,16 +449,18 @@ func (c *Cache) cleanExpiredBatch(ctx context.Context) (int64, error) {
 	return deleted, nil
 }
 
-// readFormat reads the first bytes of a source or variant file, without
-// moving its offset, and returns the format they show. Sources and variants
-// are in the same six formats.
+// readFormat reads up to magic.MaxMagicBytes first bytes of a source or
+// variant file, without moving its offset, and returns the format they show.
+// Sources and variants are in the same seven formats; a variant is SVG only
+// as orig of an SVG source. A file shorter than magic.MinMagicBytes is
+// io.EOF.
 func readFormat(content io.ReaderAt) (imagerequest.Format, error) {
-	firstBytes := make([]byte, magic.MinMagicBytes)
+	firstBytes := make([]byte, magic.MaxMagicBytes)
 
-	_, err := content.ReadAt(firstBytes, 0)
-	if err != nil {
+	n, err := content.ReadAt(firstBytes, 0)
+	if err != nil && (!errors.Is(err, io.EOF) || n < magic.MinMagicBytes) {
 		return "", fmt.Errorf("failed to read the first bytes: %w", err)
 	}
 
-	return magic.DetectFormat(firstBytes)
+	return magic.DetectFormat(firstBytes[:n])
 }