Fall back to ffmpeg when Go's JPEG decoder rejects a source

Go's stdlib image/jpeg refuses some valid-but-rare features such as
4:1:1 luma/chroma subsampling (ANN's CDN serves these). When the
in-process decode fails, route the bytes through ffmpeg the same way
we already do for video sources.

internal/web/thumbs.go

@@ -178,7 +178,7 @@ func buildAvifThumb(src, outPath string) error {
 	}
 
 	if urlIsVideo || isVideoContentType(resp.Header.Get("Content-Type")) {
-		return buildAvifFromVideoFrame(body, outPath)
+		return buildAvifFromFFmpeg(body, outPath)
 	}
 
 	if isAvif(body) {
@@ -195,6 +195,13 @@ func buildAvifThumb(src, outPath string) error {
 	}
 	src2, _, err := image.Decode(bytes.NewReader(body))
 	if err != nil {
+		// Go's stdlib JPEG decoder rejects some valid-but-rare features
+		// (e.g. 4:1:1 luma/chroma subsampling served by ANN's CDN). Hand
+		// the bytes to ffmpeg as a permissive fallback — it produces the
+		// same shape of output as the video path.
+		if ffErr := buildAvifFromFFmpeg(body, outPath); ffErr == nil {
+			return nil
+		}
 		return fmt.Errorf("decode: %w", err)
 	}
 	b := src2.Bounds()
@@ -265,12 +272,13 @@ func isVideoContentType(ct string) bool {
 	return strings.HasPrefix(strings.ToLower(strings.TrimSpace(ct)), "video/")
 }
 
-// buildAvifFromVideoFrame writes the downloaded video to a temp file, asks
+// buildAvifFromFFmpeg writes the downloaded bytes to a temp file, asks
 // ffmpeg to extract a single resized frame as PNG, then hands it to avifenc.
-// ffmpeg is optional — if it's missing we return an error and the caller
+// Used both for video sources and as a permissive fallback for images whose
-// falls back to redirecting to the source URL, just like any other build
+// JPEG features Go's stdlib decoder rejects. ffmpeg is optional — if it's
-// failure.
+// missing we return an error and the caller falls back to redirecting to
-func buildAvifFromVideoFrame(body []byte, outPath string) error {
+// the source URL, just like any other build failure.
+func buildAvifFromFFmpeg(body []byte, outPath string) error {
 	if _, err := exec.LookPath("ffmpeg"); err != nil {
 		return fmt.Errorf("ffmpeg not available: %w", err)
 	}