Support SVG sources (closes #68)

The fetcher accepts image/svg+xml and internal/magic detects SVG. orig
of an SVG at the orig size is the SVG unchanged, cached as a variant; at
another size it is ErrSVGNotOriginalSize (400). Any other format renders
it with libvips' thumbnail at the least size covering the output, at
most 8192 on a side, so it stays sharp. An SVG's own size is not
limited; one with a side over 8192 times the other is ErrSVGTooThin
(415). The cache reads up to 1024 first bytes to find a file's format.
Tests that pinned the SVG refusal now expect SVG to be read. Render time
and memory are unbounded: P1 in TODO.md until the owner decides on
#68.

Model: opus-5-5

internal/handlers/image.go

@@ -267,11 +267,19 @@ func (s *Handlers) respondImageError(
 		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat):
 		status = http.StatusUnsupportedMediaType
 		message = "unsupported source format"
+	case errors.Is(err, imageprocessor.ErrSVGTooThin):
+		status = http.StatusUnsupportedMediaType
+		message = fmt.Sprintf("SVG source too thin to render: one side is "+
+			"more than %d times the other", imageprocessor.MaxInputDimension)
 	case errors.Is(err, imageprocessor.ErrOutputTooLarge):
 		status = http.StatusBadRequest
 		message = fmt.Sprintf("size too large for the source: the side "+
 			"computed from its proportions is above %d",
 			imageprocessor.MaxOutputDimension)
+	case errors.Is(err, imageprocessor.ErrSVGNotOriginalSize):
+		status = http.StatusBadRequest
+		message = "an SVG source is returned as SVG only at the orig size; " +
+			"ask for another format to resize it"
 	case errors.Is(err, imageprocessor.ErrAVIFOutputTooLarge):
 		status = http.StatusBadRequest
 		message = fmt.Sprintf("size too large for AVIF: the output "+