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.

rust/collector/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "ping-collector"
 version = "0.1.0"
-description = "Ping River Monitor data collector: RID gauges, HII/ThaiWater, RID dams, Open-Meteo rain -> PostgreSQL"
+description = "Ping River Monitor service: collection (RID, HII/ThaiWater, RID dams, Open-Meteo), forecast, notifications and the website/API"
 edition.workspace = true
 rust-version.workspace = true
 license.workspace = true
@@ -10,8 +10,10 @@ repository.workspace = true
 
 [dependencies]
 anyhow.workspace = true
+axum.workspace = true
 chrono.workspace = true
 clap.workspace = true
+form_urlencoded.workspace = true
 log.workspace = true
 ping-forecast = { path = "../forecast" }
 reqwest.workspace = true
@@ -19,6 +21,8 @@ serde.workspace = true
 serde_json.workspace = true
 sqlx.workspace = true
 tokio.workspace = true
+tower.workspace = true
+tower-http.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true