Function sync_library 

pub fn sync_library(
    db: &Database,
    client: &SubsonicClient,
    full: bool,
    server_url: &str,
    username: &str,
) -> Result<SyncResult, SyncError>

Pull the Navidrome/Subsonic library into the local DB.

If full is false and a last_sync timestamp exists for this server, performs an incremental sync using getAlbumList2(type=newest), stopping when all albums on a page predate the last sync. Otherwise does a full sync using alphabeticalByName.

Pipeline: paginate album list -> fetch album details in parallel (rayon) -> batch-write each page in a single transaction.

Deduplication happens in upsert_track — if a local track already exists with the same artist + album + title + track#, the remote_id and remote_url are merged onto the existing row instead of creating a duplicate.