perf(redis): use sorted set index for O(log N) schedule claiming

[isimisi]

explanation: #13 (comment)

[RomainLanz]

Hi!

Thanks for the PR.

We need a migration path for existing Redis users before merging this.

With this change, claimDueSchedule() only reads from the new schedules::due ZSET, but users upgrading from the current version will only have the existing schedule hashes plus schedules::index. Those schedules will never be claimed until something manually rewrites them or calls backfillDueIndex().

Can we make the migration automatic/idempotent, or expose and document a required migration step? For example, on startup or first claim, backfill schedules::due from schedules::index for active schedules with next_run_at.

[isimisi]

Hey @RomainLanz, great point. I completely missed the migration path for existing users.

I'll add an automatic backfill that runs on startup: on first claimDueSchedule() call (or worker boot), scan schedules::index and populate schedules::due from any schedule that has a next_run_at. I'll make it idempotent so it's safe to run multiple times across worker restarts or multi-instance deployments.