fix(subsonic): update existing playlist in-place by passing playlistId - #226

Problem

In the Subsonic REST API specification, createPlaylist is the standard endpoint for both creating and replacing playlist tracklists:

• Without playlistId, the server creates a new playlist entity.
• With playlistId, the server updates the existing playlist in-place.

Previously, Explo did not pass playlistId in CreatePlaylist() and instead invoked DeletePlaylist() prior to creation. This destructively deleted the playlist entity from the database on every sync, assigning a new playlist ID and breaking client applications (e.g. Symfonium, DSub) that track playlist IDs for favorites and delta sync.

Solution

1. In CreatePlaylist(), look up the existing playlist ID via SearchPlaylist(). If found, pass createPlaylist?playlistId=...&name=... to update the playlist tracklist in-place without altering its ID.
2. In main.go, skip pre-deleting the playlist for subsonic (if cfg.ReplacePlaylist && cfg.System != "subsonic"), keeping DeletePlaylist() available on the Subsonic client interface.