- Add ScoreCache GenServer with node-local ETS table keyed by
{band, valid_time}, subscribed to "propagation:cache" PubSub topic so
every pod stays in sync with a single hourly compute
- scores_at/3 checks cache first, falls back to DB and populates on miss
- PropagationGridWorker warms and broadcasts the cache for each band
after every forecast hour upsert; prunes >2h old entries
- Replace per-pixel string-keyed Map with flat Int8Array over the CONUS
grid in propagation_map_hook.ts to eliminate allocations in the tile
rasterization hot loop (interpolateScore / propagationReach)
