Refuse SVG sources and never save an unknown input as JPEG (closes #68)

The upstream fetch no longer accepts image/svg+xml, and both image
routes answer 415, not 500, for any upstream Content-Type outside the
source formats. Before decoding, the image processor names the input's
format from govips' DetermineImageType and refuses, with 415, input
whose first bytes libvips reads as anything but the six source formats,
so an SVG passing the first-bytes check for AVIF never reaches librsvg.
orig of such an input is refused instead of being saved as JPEG.
README.md drops SVG from the source formats.

SVG detection stays in internal/magic, unused on the request path, for
#168. An AVIF image sequence
(brand avis) answers 415.

Model: opus-5-5

internal/imageprocessor/source_formats_internal_test.go

@@ -0,0 +1,46 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+// TestImageProcessor_LoadsEachSourceFormat verifies that an image in each of
+// the six source formats, saved by the processor itself, is loaded and saved
+// again in its own format (orig).
+func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
+	t.Parallel()
+
+	proc := New(Params{})
+
+	for _, format := range []Format{
+		FormatJPEG, FormatPNG, FormatGIF, FormatWebP, FormatAVIF, FormatJXL,
+	} {
+		saved, err := proc.Process(t.Context(),
+			bytes.NewReader(createTestPNG(t, 20, 20)), &Request{Format: format})
+		if err != nil {
+			t.Fatalf("saving the test image as %s: %v", format, err)
+		}
+
+		source, err := io.ReadAll(saved.Content)
+		if err != nil {
+			t.Fatalf("reading the %s image: %v", format, err)
+		}
+
+		result, err := proc.Process(t.Context(), bytes.NewReader(source),
+			&Request{Format: FormatOriginal})
+		if err != nil {
+			t.Errorf("orig of the %s image: %v", format, err)
+
+			continue
+		}
+
+		t.Logf("orig of the %s image: %s", format, result.ContentType)
+
+		if result.ContentType != FormatToMIME(format) {
+			t.Errorf("orig of the %s image is %s, want %s",
+				format, result.ContentType, FormatToMIME(format))
+		}
+	}
+}