feat: station search in side panel; DB stats cover HII tables

A search box above the station lists filters both the RID flow list and
the basin-station list by code, name (Thai included), or river, and
persists across refreshes. /api/stats now reports whole-DB totals — RID
+ hii_rainfall + hii_waterlevel counts, combined station count, and a
date range spanning all sources — with per-source breakdown fields the
stats strip shows as tile notes. Sensor section renamed to 'Additional
basin stations · ThaiWater/HII'.

tests/test_dashboard.py

@@ -35,7 +35,9 @@ def test_dashboard_loads_additional_thaiwater_sensors():
     html = DASHBOARD_PATH.read_text(encoding="utf-8")
 
     assert "fetch('/sensors/thaiwater')" in html
-    assert "Additional ThaiWater sensor" in html
+    assert "Additional basin stations" in html
+    assert 'id="station-search"' in html
+    assert "applyStationSearch" in html
 
 
 def test_dashboard_shows_hii_rainfall_layer():