fix: detect video stream by codec_type instead of codec name allowlist

GetMediaInfo previously only recognized mpeg2video/h264/hevc and silently
returned 0x0 with no error for other codecs (VC-1, MPEG-4 ASP, AV1,
ProRes), causing DetectMediaType to misclassify as DVD and feeding bogus
dimensions to the zscale filter. Pick the first stream with
codec_type=video and return an explicit error if none is found.

internal/encoder/encoder.go

@@ -27,6 +27,7 @@ type StreamInfo struct {
 	SampleAspectRatio  string `json:"sample_aspect_ratio"`
 	DisplayAspectRatio string `json:"display_aspect_ratio"`
 	CodecName          string `json:"codec_name"`
+	CodecType          string `json:"codec_type"`
 }
 
 type ProbeResult struct {
@@ -116,14 +117,19 @@ func (e *Encoder) GetMediaInfo(path string) (width, height int, interlaced bool,
 		return 0, 0, false, fmt.Errorf("parsing ffprobe output: %w", err)
 	}
 
+	foundVideo := false
 	for _, stream := range result.Streams {
-		if stream.CodecName == "mpeg2video" || stream.CodecName == "h264" || stream.CodecName == "hevc" {
+		if stream.CodecType == "video" {
 			width = stream.Width
 			height = stream.Height
-			fmt.Printf("DEBUG detected video: %dx%d, SAR=%s\n", width, height, stream.SampleAspectRatio)
+			foundVideo = true
+			fmt.Printf("DEBUG detected video: %dx%d, codec=%s, SAR=%s\n", width, height, stream.CodecName, stream.SampleAspectRatio)
 			break
 		}
 	}
+	if !foundVideo {
+		return 0, 0, false, fmt.Errorf("no video stream found")
+	}
 
 	cmd = exec.Command(e.ffmpegPath,
 		"-hide_banner",