Replace timer-based _monitor_progress (1s sleep loop) with a
ProgressReporter class that publishes on actual state changes,
throttled to at most 1 publish per 250ms. A background flush
every 2s keeps ETA/elapsed current during quiet periods.
Switch WebSocket forwarder from get_message() polling (1s timeout)
to async pubsub.listen() for instant Redis-to-WebSocket delivery.
Combined latency improvement: ~1.5s average → ~250ms.
902f1b0852