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).

src/config.py

@@ -38,6 +38,12 @@ class Config:
     TARGET_URL = "https://hyd-app-db.rid.go.th/hydro1h.html"
     API_URL = "https://hyd-app-db.rid.go.th/webservice/getGroupHourlyWaterLevelReportAllHL.ashx"
     THAIWATER_API_KEY = os.getenv("THAIWATER_API_KEY")
+
+    # Public flood notifications (ntfy). Off unless NTFY_SERVER is set.
+    NTFY_SERVER = os.getenv("NTFY_SERVER", "").strip()
+    NTFY_TOPIC_PREFIX = os.getenv("NTFY_TOPIC_PREFIX", "ping").strip()
+    NTFY_TOKEN = os.getenv("NTFY_TOKEN", "").strip()  # publish token if ACL enabled
+    PUBLIC_URL = os.getenv("PUBLIC_URL", "https://water.buildfor.life/").strip()
     REQUEST_TIMEOUT = int(os.getenv("REQUEST_TIMEOUT", "30"))
     USER_AGENT = (
         "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 "