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.
grabowski