Function sweep_retention 

pub async fn sweep_retention(
    pool: &SqlitePool,
    days: u32,
    now: DateTime<Utc>,
) -> Result<u64>

Deletes every tune run with started_at at or before the days-day cutoff (see cutoff_for), along with – via ON DELETE CASCADE – its samples, results, and write-back audit rows. Returns the number of runs deleted.

Logs at INFO when something was actually deleted (so deletions are never silent, per history-retention’s design note) and at DEBUG otherwise, so a no-op sweep – the common case for an install well under its retention window – doesn’t add log noise at the default level.