OptionalnowOptionaltableMust satisfy and match createD1Storage's max-39 tablePrefix contract.
Resolved (delivered/seen/dismissed/archived/discarded) notifications whose
updatedAt is older than this expire. PENDING rows are never touched — one
may be waiting on a future deliverAt, and reaping it would drop a signal
the model has not seen. No default: the caller (createFlowsafeWorker's purge duty)
gates this on an opt-in retention window, since a durable inbox is meant to
be readable until the host says otherwise.
Clock override for tests.