feat: rainfall + HII water-level layers on the dashboard

New endpoints GET /api/hii/rainfall/latest and /api/hii/waterlevel/latest
serve the latest per-station rows from the hii_* tables. The map gains a
TWA-style rain layer: circle markers binned by TMD 24-h classes (blues
for light/moderate/heavy, site amber/red for very-heavy/extreme), with a
legend block and show/hide toggle. The ThaiWater sensor panel now
prefers the DB-backed HII feed (no API key required, 97+ stations,
ThaiWater storage-percent situation colors, gauge-datum conversion via
offset_msl) and falls back to the live /sensors/thaiwater passthrough.

tests/test_dashboard.py

@@ -38,6 +38,19 @@ def test_dashboard_loads_additional_thaiwater_sensors():
     assert "Additional ThaiWater sensor" in html
 
 
+def test_dashboard_shows_hii_rainfall_layer():
+    html = DASHBOARD_PATH.read_text(encoding="utf-8")
+
+    assert "fetch('/api/hii/rainfall/latest')" in html
+    assert "fetch('/api/hii/waterlevel/latest')" in html
+    assert "renderRainLayer" in html
+    assert "rain-toggle" in html
+    assert "Rainfall · 24 h" in html
+    # TMD rain classes on the legend
+    assert "Heavy 35–90" in html
+    assert "Extreme > 150" in html
+
+
 def test_dashboard_loads_postgresql_history_chart():
     html = DASHBOARD_PATH.read_text(encoding="utf-8")