ListRedisScheduleSource runs a full SCAN across the entire Redis keyspace on every update_interval #122

The ListRedisScheduleSource currently runs a full SCAN across the entire Redis keyspace on every update_interval to discover past time keys. When Redis holds a large number of keys
(millions+), this becomes a significant bottleneck — it causes heavy CPU load on both Redis and the scheduler process, and the latency scales linearly with total key count regardless of how many
schedule keys actually exist.