Function push_snapshot 

pub async fn push_snapshot(
    storage: &dyn SyncStorage,
    encrypted_snapshot: Vec<u8>,
    device_id: &str,
    current_seq: u64,
    clock: &dyn Clock,
) -> Result<(), SnapshotError>

Upload a snapshot to the sync storage and update the device head.

Also uploads per-device cursor metadata (snapshot_meta.json.enc) so that bootstrapping devices know where each device was at snapshot time, and GC can safely delete only changesets covered by the snapshot.