feat(rust): ping-collector, phase 1 of the Rust rewrite

rust/collector replaces the collection half of the Python web app: RID hourly
gauges (with the late-report semantics: store everything, retry every 10 min
while RID is behind, re-fetch yesterday's holes, startup gap fill), HII rain +
water level, RID dams (today + yesterday) and Open-Meteo rain. Same tables and
upsert semantics, one UNNEST statement per table, Bangkok wall-clock time
regardless of the server time zone.

Python side: COLLECTION_ENABLED=false makes the API leader stop collecting and
instead re-forecast + notify when the newest RID reading in the database
advances (polled every FORECAST_POLL_SECONDS) and once per clock hour;
/scraping/status then serves the collector's status file; /scrape/trigger
answers 409.

Verified: 20 Rust tests (parsers, gap/schedule logic, database tests against
PostgreSQL); live parity run of both collectors into two empty databases:
all 9 tables row-for-row identical (scripts/collector_parity.py); the Python
API served health, measurements, stats, status and model forecasts from a
database filled only by ping-collector. 177 Python tests pass.

reqwest uses native-tls: app.rid.go.th offers only TLS 1.2 CBC suites, which
rustls refuses.

Deploy/cutover/rollback: scripts/install_collector.sh, scripts/ping-collector.service;
CI: .gitea/workflows/rust.yml (fmt, clippy, tests with a PostgreSQL service,
release artifact).

CLAUDE.md

@@ -28,6 +28,11 @@ Production: one systemd unit on a small VPS, `/opt/thailand-water-monitor`, user
 - Do not add Co-Authored-By trailers.
 - The dashboard is a single file, `src/static/dashboard.html`, EN + TH via the `t()`
   table: every user-visible string needs both languages.
+- Rust (`rust/`, see rust/README.md): `cargo fmt --all`, `cargo clippy --all-targets
+  -- -D warnings`, `cargo test` (database tests need `TEST_DATABASE_URL`). The Rust
+  collector writes the same tables as the Python collectors: a change to a table,
+  upsert or parser on one side must be made on the other until the Python side is
+  retired (check with a live run of both into two databases).
 
 ## Where things are
 
@@ -36,5 +41,7 @@ Production: one systemd unit on a small VPS, `/opt/thailand-water-monitor`, user
   `rain.py` (Open-Meteo), `dam.py`, `hii_rain.py`.
 - `scripts/retrain.sh` + `water-monitor-retrain.timer`: monthly retrain with staged
   promote. `scripts/dev_proxy.py`: serve the working-copy dashboard against the live API.
+- `rust/collector` (`ping-collector`): Rust data collector; with it deployed the
+  Python app runs `COLLECTION_ENABLED=false`. Rewrite plan: `docs/RUST_REWRITE.md`.
 - `docs/FLOOD_FORECASTING.md` is the authoritative model write-up; `docs/DATA_SOURCES.md`
   the source catalog.