feat: discharge-driven river animation speed, replay stat tiles, hourly doc render

- river dash animation now flows at a speed continuously derived from
  each segment's discharge (period 260/(Q+45) s, clamped 0.5-5.5 s) via
  inline per-path animation-duration, so it updates live and per-frame
  during the replay (CSS speed classes removed - setStyle cannot change
  classes)
- the replay drives the Combined discharge and Strongest flow stat
  tiles each frame (marked '2024 replay'), restored on finish
- docs: hour-by-hour detection detail render (22-28 Sep 2024) showing
  the model alert at 24 Sep 01:00, flooding at 25 Sep 01:00, and the
  24 h warning between them; embedded with commentary
- Matrix alerts now link to https://water.buildfor.life/ (override via
  ALERT_DASHBOARD_URL), replacing the Grafana public dashboard link

docs/FLOOD_FORECASTING.md

@@ -306,6 +306,16 @@ honestly: the predicted peak tops out ~0.4 m short of the actual 5.30 m record
 noisier on the recession limbs. The same model track drives the dashboard's
 "Replay Oct 2024 flood" feature, so this chart can be watched live on the map.
 
+![Hour-by-hour detail of the detection window, 22–28 September
+2024](img/backtest-2024-p1-detail.png)
+
+The hour-by-hour detail of the detection window shows the sequence exactly: the
+predicted 24 h peak (amber) starts pulling away from the observed level late on
+23 September as upstream gauges rise, the warning probability snaps from ~0 to
+1.0 at **01:00 on 24 September**, and the river crosses 3.70 m at **01:00 on
+25 September** — a clean 24-hour warning, delivered while the river in town
+still looked normal at 2.8 m.
+
 - **25 September cold start.** P.1's first warning crossing of the episode was
   alerted **24–26 hours ahead**. This is the genuinely impressive case: the river
   was in normal state, and the alert came from upstream routing alone.