harden ffmpeg pipeline failure handling and stream mapping

internal/audio/convert/convert.go

@@ -52,13 +52,13 @@ func Convert(path string, cfg config.ConversionConfig) (string, error) {
 		return path, fmt.Errorf("conversion failed: %w: %s", err, string(output))
 	}
 
-	if path != finalPath {
-		_ = os.Remove(path)
-	}
 	if err = os.Rename(tmpPath, finalPath); err != nil {
 		_ = os.Remove(tmpPath)
 		return path, err
 	}
+	if path != finalPath {
+		_ = os.Remove(path)
+	}
 
 	return finalPath, nil
 }