fix(forecast): no nightly gaps in the issued model forecasts

The P.1 chart's "24 h peak, as issued" line had a gap every night and a
dip at 21:00. Two causes:

- A run forecasts from the newest reading only. RID publishes 22:00..06:00
  in one batch in the morning (and now and then one hour late), so those
  hours were never forecast from. After each run the collector now
  forecasts the hours of the last 48 h (two weeks on the first run after a
  start) that have a RID reading but no model forecast, each as a run at
  that hour would have: gauges, HII fill and dam state up to that hour.
  Rain is fetched once, so a late hour's rain_fc24 partly holds rain that
  has since fallen (as in training). Only missing (hour, station) pairs are
  written. `ping-collector catch-up-forecasts --hours N` goes further back.
- Overnight each hourly run re-issues the 21:00 reading, after 6 h as the
  heuristic, which overwrote the 21:00 model row. A heuristic row no longer
  replaces a model row in forecast_history.

The dashboard draws only model rows on the issued line.

Checked on a local copy of 18 days of RID data: the catch-up of an hour
matches `ping-collector forecast --now <hour>`; 288 hours took 20 s
(debug build). Tests for both database rules.

docs/FLOOD_FORECASTING.md

@@ -627,6 +627,25 @@ A stored forecast older than `FORECAST_MAX_AGE_SECONDS` (2 h, i.e. the service
 has stopped forecasting) is still served, marked `X-Data-Stale: true`. See
 `rust/README.md`.
 
+**The issued history has every hour.** A run forecasts from the newest reading
+only, and RID publishes 22:00 to 06:00 in one batch in the morning (and now and
+then reports a single hour late), so those hours were never forecast from and
+the 24 h peak "as issued" had a gap every night. After each run the collector
+therefore forecasts the hours of the last 48 h (two weeks after a start) that
+have a RID reading but no model forecast, each as a run at that hour would
+have: gauges, HII fill and dam state up to that hour only. The exception is
+rain: it is fetched once, so a late hour's `rain_fc24` partly holds
+Open-Meteo's estimate of rain that has since fallen, as in training, where
+the forward rain is observed rain. Such rows are written only where no model
+row exists; `created_at` tells them apart (more than an hour after `as_of`).
+`ping-collector catch-up-forecasts --hours N` does the same further back (up
+to 60 days).
+
+A model row in `forecast_history` is never replaced by a heuristic one. While
+RID is silent overnight each hourly run re-issues the 21:00 reading, and once
+it is 6 h old (`stale_after_h`) as the heuristic; that used to overwrite the
+21:00 model forecast every night.
+
 A station without a model bundle, or whose bundle needs a feature the Rust side
 cannot build, gets the heuristic, as in Python. So **before the first training
 run `/forecast` returns 200 with an all-heuristic payload**, provided there is