The collection-leader worker now precomputes forecasts after every
scrape cycle: primes the /forecast cache (users never trigger the
multi-second inference — its TTL rises to 4500s so the hourly refresh
always wins) and persists every issued forecast to a new
forecast_history table keyed by (as_of, station, horizon) with
predicted max level, warn/danger probabilities, current level, and
model_version. This is the operational record the backtests lacked —
predicted-vs-actual becomes a simple join instead of retraining
historical models.
GET /api/forecast/history/{station} serves the archive (hours or
start/end + horizon filters, 5-min edge cache), and the station history
chart overlays 'Model 24 h peak (as issued)' as a dashed violet line
once data accumulates.
