chore(player): report marker counts when loading playback extras

The extras loader logged only the chapter count, so a user report of
"auto skip never fires" could not be told apart from "the server has no
intro marker for this item" — the two need opposite fixes. Log the
marker count and types on all three load paths, including the cache-only
one that previously logged nothing at all.

Drop PlexVideoPlaybackData.markers while here: the playback-start parse
filled it on every item and no caller ever read it, because the player
controls fetch their own PlaybackExtras.

Document why getPlaybackExtras may serve the shared metadata cache row
without a freshness check: getPlaybackInitialization refreshes that row
network-first before the controls mount. That ordering is what makes
cache-first correct, and nothing said so.

lib/models/plex/plex_video_playback_data.dart

@@ -10,8 +10,6 @@ class PlexVideoPlaybackData {
 
   final List<MediaVersion> availableVersions;
 
-  final List<MediaMarker> markers;
-
   final int selectedMediaIndex;
 
   final int selectedPartIndex;
@@ -20,7 +18,6 @@ class PlexVideoPlaybackData {
     required this.videoUrl,
     required this.mediaInfo,
     required this.availableVersions,
-    this.markers = const [],
     this.selectedMediaIndex = 0,
     this.selectedPartIndex = 0,
   });