Replaces immediate polling with queue-based scheduling:
- TaskQueue with min-heap (container/heap) for NextRun-ordered execution
- Worker goroutines that block on WaitNext() until tasks are due
- Tasks only execute when NextRun <= now, respecting adaptive intervals
- Automatic rescheduling after execution via scheduler.BuildPlan
- Queue depth tracking for backpressure-aware interval adjustments
- Upsert semantics for updating scheduled tasks without duplicates
Task 6 of 10 complete (60%). Ready for error/backoff policies.
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