Function prune_store_older_than 

pub async fn prune_store_older_than(
    store: &dyn Store,
    max_age: Duration,
) -> Result<u64>

Delete events older than max_age from store, returning the count removed. The daemon’s age-based retention policy (#4), factored out of crate::fleet::FleetManager::prune_older_than so the k8s arm — which builds no FleetManager (#83) — can prune off the shared store too.