Optimize ListRedisScheduleSource: replace SCAN with sorted set time index

[tmkarthi]

Summary

• Replace the SCAN-based discovery of past time keys with a ZRANGEBYSCORE lookup on a new {prefix}:time_index sorted set, eliminating full keyspace scans that strain Redis when there are many keys
• Track time keys in the sorted set automatically on add_schedule, and clean up stale entries (older than 5 minutes with empty lists) lazily from delete_schedule, rate-limited to once per minute to avoid excess Redis calls
• Fetch past time schedules on every get_schedules call (not just the first run) so that schedules added in a past minute after the previous call are never missed
• Pass current_time from get_schedules into _get_previous_time_schedules to prevent duplicate fetches when the minute boundary crosses between the two calls
• Add populate_time_index constructor parameter for users upgrading from older versions: set to True once to backfill the index via a one-time SCAN, then False for all subsequent runs
• Update README with new Redis key documentation and populate_time_index migration guide

Test plan

• Existing tests for cron, interval, timed schedules, removal, and migration all pass
• test_time_index_populated_on_add — verifies add_schedule writes to the sorted set
• test_time_index_not_eagerly_cleaned_on_delete — verifies no race-prone eager cleanup
• test_cleanup_removes_old_empty_entries — stale empty entries older than 5 min are removed
• test_cleanup_keeps_non_empty_entries — entries with remaining schedules are preserved
• test_cleanup_keeps_recent_empty_entries — entries within 5-min window are not removed
• test_past_schedules_found_via_time_index — past schedules discovered via sorted set
• test_populate_time_index_from_existing_keys — populate_time_index=True backfills index
• test_post_send_triggers_cleanup — full lifecycle: add → get → post_send → cleanup
• test_cleanup_rate_limited — cleanup runs at most once per minute
• test_cron_and_interval_not_in_time_index — non-time schedules don't pollute the index