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.

src/web_api.py

@@ -9,6 +9,7 @@ import secrets
 import time
 from contextlib import asynccontextmanager
 from datetime import datetime, timedelta
+from decimal import Decimal
 from threading import Lock
 from typing import Any, Dict, List, Optional
 
@@ -536,6 +537,73 @@ async def get_thaiwater_sensors():
         raise HTTPException(status_code=502, detail="ThaiWater API unavailable")
 
 
+def _hii_engine():
+    """Engine of the HII store, or None when collection is disabled."""
+    collector = app_state.get("hii_collector")
+    if not collector:
+        return None
+    store = collector.store
+    if not store.engine and not store.connect():
+        return None
+    return store.engine
+
+
+def _hii_rows(sql: str, params: Dict[str, Any]) -> List[Dict[str, Any]]:
+    """Run a read query against the hii_* tables, JSON-normalizing numerics."""
+    engine = _hii_engine()
+    if engine is None:
+        return []
+    from sqlalchemy import text
+
+    with engine.connect() as conn:
+        rows = [dict(row._mapping) for row in conn.execute(text(sql), params)]
+    for row in rows:
+        for key, value in row.items():
+            if isinstance(value, Decimal):
+                row[key] = float(value)
+    return rows
+
+
+@app.get("/api/hii/rainfall/latest")
+async def get_hii_rainfall_latest(hours: int = Query(26, ge=1, le=168)):
+    """Latest rainfall reading per HII station (Ping basin, collected hourly)."""
+    increment_counter("api_requests", labels={"endpoint": "hii_rainfall"})
+    sql = """
+        SELECT s.id AS station_id, s.oldcode, s.name_en, s.name_th,
+               s.latitude, s.longitude, s.agency,
+               m.timestamp, m.rain_1h, m.rain_24h
+        FROM hii_rainfall m
+        JOIN hii_rain_stations s ON s.id = m.station_id
+        JOIN (SELECT station_id, MAX(timestamp) AS latest_ts
+              FROM hii_rainfall WHERE timestamp >= :cutoff
+              GROUP BY station_id) latest
+          ON latest.station_id = m.station_id AND latest.latest_ts = m.timestamp
+    """
+    cutoff = datetime.now() - timedelta(hours=hours)
+    return await asyncio.to_thread(_hii_rows, sql, {"cutoff": cutoff})
+
+
+@app.get("/api/hii/waterlevel/latest")
+async def get_hii_waterlevel_latest(hours: int = Query(26, ge=1, le=168)):
+    """Latest water-level reading per HII station (Ping basin, m MSL)."""
+    increment_counter("api_requests", labels={"endpoint": "hii_waterlevel"})
+    sql = """
+        SELECT s.id AS station_id, s.oldcode, s.rid_code, s.name_en, s.name_th,
+               s.latitude, s.longitude, s.agency, s.river_name,
+               s.offset_msl, s.min_bank_msl, s.is_key_station,
+               m.timestamp, m.wl_msl, m.discharge, m.storage_percent,
+               m.situation_level, m.diff_wl_bank
+        FROM hii_waterlevel m
+        JOIN hii_wl_stations s ON s.id = m.station_id
+        JOIN (SELECT station_id, MAX(timestamp) AS latest_ts
+              FROM hii_waterlevel WHERE timestamp >= :cutoff
+              GROUP BY station_id) latest
+          ON latest.station_id = m.station_id AND latest.latest_ts = m.timestamp
+    """
+    cutoff = datetime.now() - timedelta(hours=hours)
+    return await asyncio.to_thread(_hii_rows, sql, {"cutoff": cutoff})
+
+
 @app.get("/measurements/history/{station_code}")
 async def get_postgres_history(
     station_code: str,