tune nats jetstream consumer flow control with ackwait, maxackpending, and maxdeliver to prevent redelivery storms

Steps

1. Set AckWait to the expected processing time so messages aren't redelivered while still being legitimately processed.
2. Set MaxAckPending to bound the number of outstanding unacknowledged messages; lower it for slow or expensive handlers, raise it for high-throughput lightweight handlers.
3. Configure MaxDeliver to cap redelivery attempts so a poison message doesn't loop forever, pairing it with a dead-letter-style handling path.
4. Optionally configure a BackOff sequence of durations, which overrides a static AckWait, so retries space out progressively instead of firing at a fixed interval.
5. Watch for consumer starvation symptoms, such as throughput flatlining despite healthy consumers, which usually indicate MaxAckPending is set too low relative to processing latency.