Stop exhausted schedules from dispatching

[mostlydev]

Summary

• Treat cron schedules with no future fire time as exhausted instead of immediately due.
• Keep exhausted schedules out of the dispatch loop and omit next_fire_at instead of serializing zero time.
• Normalize persisted zero time pointers in schedule state so old bad state stops leaking as 0001 timestamps.

Tests

• go test ./cmd/claw-api ./cmd/claw