The supervisor can now pause dequeuing - and freeze consumer-pool
scale-up - when a backpressure signal says the cluster can't place more
work, then ramp dequeuing back up gradually once it clears. The signal
is a verdict published to a Redis key by a cluster-side component; the
supervisor reads it on a short refresh and gates `preDequeue` on it.
Off by default (`TRIGGER_DEQUEUE_BACKPRESSURE_ENABLED`). Everything
fails open: a missing, stale, or unreadable verdict never pins the
brake, and the hot-path read is a synchronous cached lookup with no I/O.
The scale-up freeze leaves scale-down untouched, and on release the
resume is ramped so a deep queue isn't hammered all at once.
Dry-run is on by default (`TRIGGER_DEQUEUE_BACKPRESSURE_DRY_RUN`): even
once enabled it only logs what it would have done, and surfaces the
computed state through metrics, until explicitly set to act. Prometheus:
`supervisor_backpressure_engaged`, `_dry_run`,
`_skipped_dequeues_total`.
Refs TRI-5354
nicktrn
35c56f1d09