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

src/alerting.py

@@ -155,9 +155,9 @@ class MatrixNotifier:
         if alert.message:
             message += f"\n**Details:** {alert.message}\n"
 
-        # Add Grafana public dashboard link
-        grafana_url = "https://metrics.b4l.co.th/public-dashboards/655730aa044f44f49b355d01386018ca"
-        message += f"\n📈 **View Dashboard:** {grafana_url}"
+        # Add live dashboard link
+        dashboard_url = os.getenv("ALERT_DASHBOARD_URL", "https://water.buildfor.life/")
+        message += f"\n📈 **View Dashboard:** {dashboard_url}"
 
         return self.send_message(message)