feat(plex): migrate video transcoding to HLS

lib/utils/codec_utils.dart

@@ -43,9 +43,9 @@ class CodecUtils {
     };
   }
 
-  /// Image-based (bitmap) subtitle codecs. These can't be converted to text;
-  /// during an HTTP/MKV transcode Plex copies the stream into the container and
-  /// the player renders it natively.
+  /// Image-based (bitmap) subtitle codecs. Plex burns these into the video
+  /// when the selected output transport cannot carry a bitmap subtitle
+  /// rendition.
   static bool isImageSubtitleCodec(String? codec) {
     if (codec == null) return false;
     return switch (codec.toLowerCase()) {
@@ -61,10 +61,9 @@ class CodecUtils {
     };
   }
 
-  /// Subtitle codecs that can be carried inside the HTTP/MKV transcode stream
-  /// (`subtitles=embedded`): text codecs plus the image codecs the MKV target
-  /// supports. Keyed sidecars are delivered separately and are not covered here.
-  static bool isEmbeddableSubtitleCodec(String? codec) {
+  /// Subtitle codecs Plex can deliver in a transcode. Text codecs can become
+  /// segmented HLS WebVTT; image codecs can be burned into the video.
+  static bool isTranscodableSubtitleCodec(String? codec) {
     return isTextSubtitleCodec(codec) || isImageSubtitleCodec(codec);
   }