feat(web): the website and API in ping-collector; Matrix gets the ntfy transitions

Phase 3 of docs/RUST_REWRITE.md: rust/collector/src/web (axum) serves what
the FastAPI app (src/web_api.py) served, so one process collects, forecasts,
notifies and serves. With WEB_LISTEN=0.0.0.0:8000 it runs inside
`ping-collector run`; `ping-collector serve` runs it alone.

Same routes, parameters, JSON fields and field order, status codes,
{"detail": ...} errors and pydantic-style 422 lists, Cache-Control, gzip
(>500 B), CORS, Umami API tracking, the dashboard and static files, and the
stale-while-revalidate response caches (now with byte budgets: a history
response can be 10 MB). /docs is the same Swagger UI (+ /redoc); its OpenAPI
document is built from the route table, whose per-route parameter
declarations also validate the requests, so docs and behaviour cannot drift.
Python output conventions are kept: isoformat(), round(), numpy's pairwise
sums (catchment means), Python's summation order (skill metrics).

scripts/api_parity.py compares the two servers route by route on one
database (53 cases incl. errors, admin and static files; query values vary
per run so FastAPI's caches answer fresh). Locally: 53 identical; the
station CRUD writes byte-identical stations.json files; the skill/trend
logic matches on 9 station/horizon pairs with synthetic model versions.
Intended differences: /health counts the ThaiWater telemetry of the same
gauges while RID lags (as the notifier); /scraping/status adds the collector
version and last forecast run; an old stored forecast is served marked
X-Data-Stale instead of recomputed; /sensors/thaiwater is cached a minute;
.geojson is served as application/geo+json. Load (local, 16 clients): the
dashboard's routes 1375 vs 552 req/s, uncached history 714 vs 53 req/s;
25 MB resident when serving only.

Matrix (MATRIX_*): the notifier sends the room the transitions ntfy gets
(per station, outlook, feed; not ntfy's per-topic summaries), each once,
with its own matrix: keys in notification_state, seeded from ntfy's on first
use so nothing already announced repeats. Checked against a fake homeserver.
It replaces the `run.py --alert-check` cron job (repeated alerts every 15
min), which install_collector.sh now comments out (crontab backup kept).

install_collector.sh: --check-api (ping-collector serve on :8001 next to
FastAPI, api_parity.py), --cutover-api (refuses on a difference; sets
WEB_LISTEN, stops and disables water-monitor.service), --rollback-api;
--rollback-forecast and --rollback bring the Python app back as server.
ping-collector.service: MemoryMax 512M. Config::from_vars for tests; route
tests against a throwaway schema.

docs/RUST_REWRITE.md

@@ -1,7 +1,8 @@
 # A Rust rewrite: assessment
 
 Status: **in progress.** Phase 1 (collector) is in production since 2026-10-17,
-phase 2 (forecast + ntfy) since 2026-10-18; see
+phase 2 (forecast + ntfy) since 2026-10-18; phase 3 (website/API + Matrix) is
+built and parity-checked, cutover pending; see
 [rust/README.md](../rust/README.md). Spike: [spikes/rust-forecast](../spikes/rust-forecast/README.md).
 
 ## Short version
@@ -85,7 +86,7 @@ This slows model iteration, which is the most active part of the repo.
 | 0 | Spike: P.1 features + HGB inference | done, bit-identical |
 | 1 | `ping-collector`: RID, HII, dams, Open-Meteo | in production since 2026-10-17 23:09 (Python: `COLLECTION_ENABLED=false`) |
 | 2 | Forecast crate + ntfy | in production since 2026-10-18 15:08 (`FORECAST_IN_COLLECTOR=true`); live check on production data: 48/48 rows identical, 96/96 probabilities bit-identical |
-| 3 | Read API | |
+| 3 | Website + read API in `ping-collector`, Matrix like ntfy | built 2026-10-18; locally 53 of 53 compared routes identical to FastAPI (`scripts/api_parity.py`); cutover with `install_collector.sh --cutover-api` |
 | 4 | Cleanup | |
 
 0. **Spike: done.** P.1 features + HGB inference, bit-identical.
@@ -106,14 +107,36 @@ This slows model iteration, which is the most active part of the repo.
    NaN at serving time (docs/FLOOD_FORECASTING.md section 5). The ntfy change:
    ThaiWater mirror readings keep level alerts and the feed check current
    while RID lags at night. The Python API then only serves.
-3. **Read API** (~2k LOC): routes recorded from the Python API as golden files,
-   then a Caddy switch. Python keeps only training/evaluation.
-4. **Cleanup:** drop the multi-backend adapters, demo databases and the
-   Python web stack.
+3. **Website + read API** (`rust/collector/src/web`, axum): every FastAPI
+   route with the same parameters, JSON fields and order, status codes,
+   FastAPI-style errors (`{"detail": ...}`, pydantic's 422 list), cache
+   headers, gzip, CORS, Umami API tracking and the stale-while-revalidate
+   response caches (now with byte budgets). `/docs` is the same Swagger UI;
+   its OpenAPI document is generated from the route table whose parameter
+   declarations also do the validation. One process serves, collects,
+   forecasts and notifies (`WEB_LISTEN`), so the status file, the leader
+   election and the `forecast_current` hand-off between processes are gone
+   in effect. Checked by `scripts/api_parity.py`, which runs `ping-collector
+   serve` next to the FastAPI app on the same database and compares every
+   route (query values vary per run so FastAPI's caches answer fresh);
+   `--cutover-api` refuses to switch on a difference. Intended differences:
+   `/health` counts ThaiWater telemetry of the same gauges while RID lags (as
+   the notifier); `/scraping/status` adds the collector version and the last
+   forecast run; `/forecast` serves an old stored forecast marked
+   `X-Data-Stale` instead of recomputing (same process); `/sensors/thaiwater`
+   is cached for a minute; database errors serve the last good response
+   where FastAPI answered `[]`. Matrix: the notifier's transitions go to the
+   room too (per station, outlook, feed; not the per-topic summaries), with
+   their own `matrix:` state keys seeded from ntfy's, replacing the
+   `run.py --alert-check` cron job that repeated alerts every 15 minutes.
+   Python keeps training, evaluation and the monthly retrain timer.
+4. **Cleanup:** drop FastAPI/uvicorn, the Python collector, notifier, CLI and
+   the multi-backend adapters (SQLite, MySQL, InfluxDB, VictoriaMetrics) and
+   demo databases; the Python environment shrinks to the training stack.
 
-Open questions before phase 3: is Matrix alerting still used? Must `/docs`
-(OpenAPI UI) stay? Is SQLite dev mode needed? Where does the monthly retrain
-run if the server stops having a Python environment for the API?
+Decided before phase 3 (2026-10-18): Matrix stays and behaves like ntfy;
+`/docs` stays; SQLite dev mode goes; the monthly retrain stays on the same
+machine (Python, `water-monitor-retrain.timer`).
 
 ## Cheaper alternative (stay Python)