feat(collector): forecast and ntfy in ping-collector; ThaiWater keeps alerts current at night

Phase 2b/2c of docs/RUST_REWRITE.md. With FORECAST_IN_COLLECTOR=true (shared
.env) ping-collector, after each cycle that brought newer RID or HII readings
and at least hourly: loads the last 336 h (RID + HII mirror fill), fetches the
rain forecast, runs every exported model (rust/forecast), stores
forecast_history and forecast_current, and sends the ntfy notifications.
The Python API then serves forecast_current as /forecast (computing in-process
only when it is older than FORECAST_MAX_AGE_SECONDS) and neither forecasts nor
notifies.

ntfy (collector/src/notify.rs): a port of src/notify.py with the same topics,
texts, hysteresis, capacity guard and notification_state keys, checked word
for word on 751 messages generated by Python (testdata/gen_notify_cases.py).
The change: for stations whose ThaiWater twin is the same sensor, a reading
newer than RID's drives the level alerts (marked as ThaiWater telemetry), and
the feed counts as stale only when neither source has a reading for 3 h. RID
alone lagging is reported once after RID_STALE_ALERT_HOURS (18). This ends the
nightly "gauge feed stale" / "recovered" pairs caused by RID's late reports.

Commands: `forecast [--json] [--save] [--now] [--rain-file]`,
`notify --dry-run`. scripts/forecast_parity.py compares Python and the
collector on the live database (same moment, same rain); locally 48/48 rows
identical. install_collector.sh: models export + that check on every install,
--cutover-forecast (refuses on a failed check) and --rollback-forecast.

Trees are packed into 24-byte nodes: 70 MB peak with all 15 models (was 140).
Forecast values are written to NUMERIC as shortest round-trip text, so they
round as psycopg2's do. DB test schemas get a counter (parallel tests collided
on Windows' coarse clock).

rust/forecast/src/hii.rs

@@ -55,6 +55,8 @@ pub struct FillInfo {
     pub station_code: String,
     pub offset: f64,
     pub filled_hours: usize,
+    /// The newest filled hour and its level.
+    pub latest: Option<(NaiveDateTime, f64)>,
 }
 
 /// pandas `Series.median` (numpy: mean of the two middle values when even).
@@ -117,6 +119,11 @@ pub fn fill(rid: &[Row], hii: &[HiiRow]) -> (Vec<Row>, Vec<FillInfo>) {
                 station_code: code.to_string(),
                 offset,
                 filled_hours: fills.len() - before,
+                latest: fills
+                    .iter()
+                    .skip(before)
+                    .max_by_key(|r| r.timestamp)
+                    .map(|r| (r.timestamp, r.water_level)),
             });
         }
     }