feat: public flood notifications over self-hosted ntfy

Anyone can now get push alerts on their phone without an account: the
monitor publishes to an ntfy server (one Go binary, ~30 MB RSS) and
subscribers pick topics in the free iOS/Android/web app.

Semantics are transitions, never state. One message when a gauge crosses
its warning or danger threshold, one all-clear when it drops back (0.10 m
hysteresis), nothing while it sits above. A three-day flood is two
messages; a quiet season is zero. Topics: ping-warning / ping-danger
(basin digest), ping-<station>-warning / -danger, ping-p1-outlook (opt-in:
model P(warning within 24 h) at P.1 rises through 50 %, clears below 25 %,
message says it is experimental), ping-status (feed stale >= 3 h /
recovered). Priority 5 on danger so it rings through Do Not Disturb.

src/notify.py runs once per collection cycle in the API process (leader
only, after the forecast precompute, same data the dashboard shows).
Last-sent state lives in a notification_state table so a restart never
re-sends; a failed publish leaves state untouched so the crossing is
retried next cycle instead of lost. Off unless NTFY_SERVER is set.

Dashboard: a "Get alerts" button (only when configured) opens a panel
with the server, per-topic cards, ntfy:// deep links and web links, app
store links and a disclaimer. EN + TH. GET /api/notifications feeds it.

scripts/install_ntfy.sh: .deb install, server.yml (loopback listen,
anonymous read, token-only write scoped to ping-*, 72 h cache, signup/
login/metrics off, tight visitor limits), systemd, user + token, .env.
Verified against ntfy 2.28.0: anon publish 403, token publish 200, token
on foreign topic 403, anon read 200, and a seeded crossing through the
real _notify_transitions path arrived in the topic with priority, tags,
click and action button. docs/NOTIFICATIONS.md has the deployment and
reverse-proxy notes. Tests: 10 for the state machine (159 total).

README.md

@@ -36,7 +36,11 @@ background in [Teaching a Model to See the Ping River Rise 13 Hours Early](https
 - **Shows it.** A Leaflet map with the river drawn as OSM geometry and styled by live
   discharge, rain gauges, the Chiang Mai inundation zones, per-station history, the
   forecast card, a replay of the 2024 flood, English/Thai, light/dark.
-- **Alerts** (optional) to a Matrix room when a gauge crosses its thresholds.
+- **Notifies.** Public push alerts over a self-hosted [ntfy](https://ntfy.sh): one
+  message when a gauge crosses its warning or danger level, one all-clear on the
+  way down, an opt-in early-warning topic from the model, nothing in between.
+  Subscribe from the free app, no account. Matrix room alerts for a team are
+  also supported.
 
 ## Quick start
 
@@ -79,6 +83,8 @@ Asia/Bangkok wall-clock without an offset suffix.
 | `GET /api/forecast/history/{code}?hours=N&horizon=24` | Forecasts as issued, for auditing lead time after the fact |
 | `GET /api/hii/rainfall/latest`, `/api/hii/waterlevel/latest` | Latest ThaiWater/HII gauge readings |
 | `GET /api/hii/rainfall/catchment?days=N` | HII gauge catchment-mean rain next to the Open-Meteo series the model uses |
+| `GET /api/forecast/skill?station_code=P.1` | Issued forecasts vs what happened, per deployed model version |
+| `GET /api/notifications` | ntfy server and topic names for the subscribe panel |
 | `GET /api/stats` | Row counts per source, date range, coverage |
 | `GET /health` | DB / upstream / memory checks |
 
@@ -122,7 +128,8 @@ models/              trained bundles + metrics.json (gitignored) and evaluation
 - [docs/DATA_SOURCES.md](docs/DATA_SOURCES.md) — every ingested and candidate source, endpoints, quirks
 - [docs/STATION_MANAGEMENT_GUIDE.md](docs/STATION_MANAGEMENT_GUIDE.md) — adding/editing gauges
 - [docs/DATABASE_DEPLOYMENT_GUIDE.md](docs/DATABASE_DEPLOYMENT_GUIDE.md), [POSTGRESQL_SETUP.md](POSTGRESQL_SETUP.md) — database setup
-- [docs/MATRIX_QUICK_START.md](docs/MATRIX_QUICK_START.md) — alert delivery
+- [docs/NOTIFICATIONS.md](docs/NOTIFICATIONS.md) — public push alerts: topics, semantics, ntfy deployment
+- [docs/MATRIX_QUICK_START.md](docs/MATRIX_QUICK_START.md) — Matrix room alerts for a team
 - [docs/GAP_FILLING_GUIDE.md](docs/GAP_FILLING_GUIDE.md) — data integrity tooling
 - [docs/references/NOTABLE_DOCUMENTS.md](docs/references/NOTABLE_DOCUMENTS.md) — official Thai government resources
 - Public overview: [buildfor.life/docs/tooling/ping-river-monitor](https://buildfor.life/docs/tooling/ping-river-monitor/)