Function set_private_marker 

async fn set_private_marker(
    services: &Services,
    room_id: &RoomId,
    user_id: &UserId,
    event: &EventId,
    thread: &ReceiptThread,
) -> Result<bool>

Resolves event to its timeline position and stores the private read marker for thread there.

Returns whether the marker advanced. A backfilled event carries no forward position, so it is rejected rather than stored.