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).

.env.example

@@ -58,6 +58,10 @@ RID_POLL_OFFSET_MINUTES=5
 RID_RETRY_MINUTES=10
 # Days of RID holes the leader re-fetches at startup (0 = off)
 GAP_FILL_DAYS_ON_START=14
+# false when the Rust ping-collector service collects (rust/collector);
+# share the status file between both services for /scraping/status
+COLLECTION_ENABLED=true
+# SCRAPING_STATUS_FILE=/opt/thailand-water-monitor/collector-status.json
 REQUEST_TIMEOUT=30
 MAX_RETRIES=3
 RETRY_DELAY_SECONDS=60