fix(notify): no "gauge feed stale" for RID's nightly pause on a quiet river

ping-status sent "gauge feed stale" around midnight and "feed recovered"
around 07:00 on each of the last three nights (ntfy and, since phase 3, the
Matrix room). RID publishes no hourly report overnight: the 21:00 reading is
the last until the 22:00..06:00 hours arrive together (06:35, 06:55, 07:22 on
2026-10-17/18/19). The ThaiWater overlay was meant to bridge that, but
ThaiWater's copies of the RID gauges come from the same feed with the same
delay (P.1's mirror still has no value for 22:00-00:00 this morning), so it
does not.

Between 21:00 and FEED_NIGHT_UNTIL_HOUR (default 9, 0 = off) the feed of a
quiet river is no longer reported stale; one still without new readings at
09:00 is. While a station is at warning/danger or the P.1 outlook is on, the
3 h rule holds all night. A feed already stale stays stale through the night
(no false "recovered"), and a real recovery is announced at any hour. The
replay of the Python notifier runs with the window off. Docs no longer claim
that ThaiWater keeps the feed current at night.

rust/forecast/src/hii.rs

@@ -2,8 +2,9 @@
 //!
 //! For stations whose HII twin is the same sensor, hours missing from the RID
 //! series (or carrying no level) are filled from HII, shifted by the median
-//! offset between the two series over their overlap. This is also what keeps
-//! the forecast current at night while RID's report lags.
+//! offset between the two series over their overlap. ThaiWater relays RID's
+//! own telemetry, so this repairs holes in RID's report but cannot bridge
+//! RID's overnight pause (the mirror arrives with the same delay).
 
 use crate::config::config;
 use crate::grid::{Row, floor_hour};