Improve MP4 movflags for better mpv seeking compatibility

- Add frag_keyframe for fragment-based seeking
- Add default_base_moof for proper base decode time
- Keep faststart for metadata at file start
- Fixes seeking/timestamp issues in mpv player

MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs

@@ -7494,7 +7494,8 @@ namespace MediaBrowser.Controller.MediaEncoding
                 else
                 {
                     // Progressive download - use faststart for proper seeking and duration
-                    format = " -f mp4 -movflags +faststart";
+                    // Use frag_keyframe for better seeking compatibility with mpv
+                    format = " -f mp4 -movflags frag_keyframe+faststart+default_base_moof";
                 }
             }