DEBUG ASSERT in PlaylistDAO::appendTracksToPlaylist()

### Bug Description

just experienced this with a branch base don main: 
```
critical [Main] DEBUG ASSERT: "playlistExists(playlistId)" in function bool PlaylistDAO::appendTracksToPlaylist(const QList<TrackId>&, int) at ./src/library/dao/playlistdao.cpp:528
```
This issue shall be a reminder to have a closer look. 


### Version

2.6-beta

### OS

_No response_