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.

tests/test_dashboard.py

@@ -139,6 +139,8 @@ def test_history_reaches_today():
     html = _html()
     assert "hours=${hours}&limit=100000" in html
     assert "/api/forecast/history/" in html
+    # the issued line is the model's; a heuristic row (the level itself) dips
+    assert "r.predicted_max_level != null && r.source === 'model'" in html
 
 
 def test_skill_loads_only_when_opened():