feat(p1): city flood stages and zones from CMU Water Center's 2024 flood map

CMU Water Center's Chiang Mai flood hazard map (CMFlood) replaces the older
municipal map. It is built on the October 2024 flood and has the Ping leave
its banks above P.1 4.20 m, with five stages: 4.30, 4.50, 4.70, 5.00 and
5.30 m (the 2024 record). The old map's seven stages (3.70-4.60 m) said the
city floods from 3.70 m; CMU calls that map out of date.

- P1_FLOOD_STAGES: the five CMU stages, discharge on RID's 2024 rating.
  /forecast's P.1 `stages` now has five entries (same format). Thresholds
  are unchanged: 4.20 m danger is the bank top, 3.70 m warning half a metre
  below (as on CMU's gauge page). flood_config.json and the Rust parity
  fixtures regenerated (only the stages changed).
- flood-zones-p1.geojson: CMU's KML (L1..L5) as five disjoint zones, zone k
  floods first at stage k; scripts/build_flood_zones.py rebuilds it.
- Dashboard: CMU's colours and credit (map attribution); the stage grid follows
  the number of stages; zone lookup for markers honours holes. The city
  verdict uses statusOf's classes: flooding (over the banks) from 4.20 m,
  high water from 3.70 m. Zones show by themselves when the river is over its
  banks or stage 1 is >= 50 % likely. Alert, replay and verdict texts no
  longer call 3.70 m city flooding (EN + TH).
- Docs: DATA_SOURCES 5.1 (CMFlood: the map, flood marks, poles, city gauges),
  FLOOD_FORECASTING, DASHBOARD_REDESIGN, NOTIFICATIONS, README credits.

docs/DASHBOARD_REDESIGN.md

@@ -52,7 +52,7 @@ One screen, no page scroll on a desktop:
 | Top bar | brand · **Chiang Mai verdict** (P.1 level + 24 h forecast, always visible, click selects P.1) · gauges reporting · highest 24 h flood chance · data age · EN/ไทย · light/dark · Get alerts |
 | Left | two lists: **RID gauges** (status bar, code, name, margin to the warning level, 72 h sparkline, level and 6 h change; sort by river order, closest to the warning level, rising) and **Basin stations** (ThaiWater/HII: % of bank height, river and agency; sort fullest first or by name); filter; legend |
 | Centre | map on a self-hosted vector basemap (labels in the page language): Ping mainstem and tributaries, gauges coloured by status with code + level labels, layers Basin stations (diamonds), Rain 24 h and Flood zones, the team's markers from zoom 12, one legend |
-| Right | selected RID gauge: level, 6 h change, discharge, warning/danger level with margins, capacity; level scale with now and 24 h peak; history (3 d / 7 d / 30 d / 90 d / 1 y / all or dates: level, discharge, the 24 h peaks as issued, warning/danger bands, hover values; up to 7 d it ends in the current forecast); forecast table (peak, chance of warning/danger at 6/12/24 h, model, issue time, disclaimer); for P.1 the seven inundation stages, the zones toggle and "How accurate is the forecast?"; data links. A basin station shows level (m MSL), % of bank height with ThaiWater's class, distance to and height of the bank top, gauge reading, agency (latest reading only: no history is kept for them) |
+| Right | selected RID gauge: level, 6 h change, discharge, warning/danger level with margins, capacity; level scale with now and 24 h peak; history (3 d / 7 d / 30 d / 90 d / 1 y / all or dates: level, discharge, the 24 h peaks as issued, warning/danger bands, hover values; up to 7 d it ends in the current forecast); forecast table (peak, chance of warning/danger at 6/12/24 h, model, issue time, disclaimer); for P.1 the five city flood stages, the zones toggle and "How accurate is the forecast?"; data links. A basin station shows level (m MSL), % of bank height with ThaiWater's class, distance to and height of the bank top, gauge reading, agency (latest reading only: no history is kept for them) |
 | Status bar | data credits, "2.7M readings since 2018" (opens the data statistics), API, source code |
 
 **Narrower screens:**
@@ -104,11 +104,11 @@ One screen, no page scroll on a desktop:
   | Danger | level ≥ `threshold_danger` |
   | No data | no reading for 24 h |
 
-- **City verdict:** the old dashboard's rule — flooding from P.1 ≥ 3.70 m; "likely" at danger ≥ 50 % or warning ≥ 70 %; watch at warning ≥ 20 % or level ≥ 3.20 m.
+- **City verdict:** the classes of `statusOf` for P.1 — flooding (the Ping over its banks) from 4.20 m, high water from 3.70 m; "flooding likely" at danger ≥ 50 %, "high water likely" at warning ≥ 70 %; watch at warning ≥ 20 % or level ≥ 3.20 m. (Until 2026-10-25 it said flooding from 3.70 m, after the older municipal map.)
 - **Data age:** "feed stale" after 3 h, except during RID's overnight pause (21:00 to 09:00 with an evening reading). The pause shows as "overnight pause", the same rule as the notifier's `FEED_NIGHT_UNTIL_HOUR`.
 - **Discharge:** comes with RID's report. When the newest hour is a level-only reading, the panel shows the last RID discharge with its time ("at 14:00").
 - **Rain layer:** off by default unless any station has heavy rain (> 35 mm / 24 h); an explicit choice is remembered.
-- **Flood zones:** shown automatically while P.1 is at or above a stage, unless hidden by the user.
+- **Flood zones:** `src/static/flood-zones-p1.geojson`, CMU Water Center's flood hazard map of the October 2024 flood (built by `scripts/build_flood_zones.py` from CMU's KML): five disjoint zones, zone k floods first when P.1 reaches stage k (4.30, 4.50, 4.70, 5.00, 5.30 m), in CMU's colours from deep red to yellow, credited in the map attribution. Shown automatically while P.1 is over its banks (danger level) or stage 1 is at least 50 % likely within 24 h, unless hidden by the user. A point's zone (team markers) honours the holes: dry islands inside a zone are outside it.
 - **Basin stations:** ThaiWater's classes for the level as a share of the bank height (critically low ≤ 10 %, low ≤ 30 %, normal ≤ 70 %, high ≤ 100 %, over the bank). On the map and in the list over the bank is danger, high is watch, the rest normal; no data after 24 h. Gauges RID already lists are shown once, as RID gauges. The layer is off until the list is opened or the button pressed.
 - **History:** above 2,500 readings (30 d and longer for most gauges) the chart shows the daily mean with the daily low–high band, so a flood peak is not averaged away; gaps longer than 3.5 h (2.5 days when daily) break the line rather than bridging RID's nightly pause. Short ranges reload with every refresh, long ones at most hourly. "All" asks for `limit=100000`: `/measurements/history` returns the oldest rows first and stops at its 50,000 default, which made the old dashboard's "All time" for P.1 end in October 2024.
 - **Issued forecasts:** the history's violet line is the model's 24 h peak for every hour (`/api/forecast/history`, `source = model` only: a heuristic row is the level itself and drew a dip into the line). The nightly gaps were RID's overnight batch; the collector now forecasts those hours when they arrive (docs/FLOOD_FORECASTING.md, Serving).