feat(train): workouts' weather from Open-Meteo, through the instance (ADR 0036)

A workout with a track gets the weather over it as its page opens, once
while it has none: the hours from the one it started in to the one it
ended in, kept in its sealed payload (temperature, feels-like, humidity,
precipitation, the WMO code, the wind, its gusts and direction) and shown
under its totals with the link to Open-Meteo its licence asks for.

Train's server asks Open-Meteo in the person's place (the `weather`
endpoint), so Open-Meteo never sees their address: the place rounded to
two decimals (also on the phone), a span of at most 48 hours, the
forecast API for recent spans and the archive for older ones, for people
signed in, at most 30 requests a person a minute, nothing kept or
logged. On by default; `WEATHER=off` switches it off, and
`OPEN_METEO_API_KEY` moves it to the paid hosts for an instance used
commercially.

Health Connect's fingerprint leaves the weather out, so keeping it
writes nothing again. No table and no migration.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/sidecars/README.md

@@ -378,6 +378,11 @@ Steps, in order; ✱ marks a step as root.
 7. **Check it**: `https://train.halkyon.eu/` answers; in Train, signed in with a Learn
    account, a route plans between two points of the region.
 
+Workouts' weather (Halkyon Train's ADR 0036) comes from Open-Meteo through Train's
+server, on by default, the free API from the server's address (non-commercial use alone,
+10,000 requests a day). `WEATHER=off` in `deploy/.env` stops it; an instance with ads or
+subscriptions needs one of Open-Meteo's paid plans, its key in `OPEN_METEO_API_KEY`.
+
 
 ## Train's map