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_hii_collector.py

@@ -275,6 +275,58 @@ class TestParseGraphRows:
         assert parse_graph_rows({}) == []
 
 
+class TestHiiApiEndpoints:
+    """Call the endpoint coroutines directly (the venv's httpx/starlette
+    combination is incompatible with TestClient)."""
+
+    @pytest.fixture
+    def web_api(self, tmp_path, monkeypatch):
+        from src import web_api
+        from src.hii_collector import HiiCollector
+
+        collector = HiiCollector(
+            {"type": "sqlite", "connection_string": f"sqlite:///{tmp_path}/api.db"}
+        )
+        now = datetime.datetime.now().replace(microsecond=0)
+        rain = parse_rain_records(_rain_payload())
+        wl = parse_waterlevel_records(_waterlevel_payload())
+        for record in rain + wl:
+            record["timestamp"] = now
+        assert collector.store.save_rain(rain) == 1
+        assert collector.store.save_waterlevel(wl) == 2
+        monkeypatch.setitem(web_api.app_state, "hii_collector", collector)
+        return web_api
+
+    def test_rainfall_latest(self, web_api):
+        import asyncio
+
+        rows = asyncio.run(web_api.get_hii_rainfall_latest(hours=26))
+        assert len(rows) == 1
+        assert rows[0]["oldcode"] == "CHM005"
+        assert rows[0]["rain_24h"] == 49.6
+        assert rows[0]["latitude"] == pytest.approx(19.12207)
+
+    def test_waterlevel_latest(self, web_api):
+        import asyncio
+
+        rows = asyncio.run(web_api.get_hii_waterlevel_latest(hours=26))
+        assert len(rows) == 2
+        p1 = next(r for r in rows if r["oldcode"] == "P.1")
+        assert p1["rid_code"] == "P.1"
+        assert p1["wl_msl"] == 303.27
+        assert p1["offset_msl"] == 300.5
+        assert p1["situation_level"] == 4
+
+    def test_empty_when_collector_disabled(self, monkeypatch):
+        import asyncio
+
+        from src import web_api
+
+        monkeypatch.setitem(web_api.app_state, "hii_collector", None)
+        assert asyncio.run(web_api.get_hii_rainfall_latest(hours=26)) == []
+        assert asyncio.run(web_api.get_hii_waterlevel_latest(hours=26)) == []
+
+
 class TestBackfillHelpers:
     def test_chunk_date_range(self):
         chunks = chunk_date_range(