PROP_ROLE=backfill drops the hot-path propagation/commercial/solar
queues and the Cron plugin. Paired with a dedicated Deployment
(nodeSelector: prop-backfill=true, 4Gi memory, no web) this lets 8GB
T620 workers drain the hrrr/weather/terrain/iemre/narr backfill
queues without competing with the hourly grid chain.
