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>

apps/train/halkyon_train_flutter/test/support/fake_weather_gateway.dart

@@ -0,0 +1,24 @@
+import 'package:halkyon_train_flutter/src/feature/weather/data/weather_gateway.dart';
+import 'package:halkyon_train_flutter/src/feature/weather/model/workout_weather.dart';
+
+/// The instance's weather as a test wants it (ADR 0036): [answer] for every
+/// span, or [WeatherUnavailable] while [failing]; what it was asked.
+class FakeWeatherGateway({var List<WeatherHour> answer = const []})
+    implements WeatherGateway {
+  bool failing = false;
+  final asked = <({double lat, double lon, DateTime from, DateTime to})>[];
+
+  @override
+  Future<List<WeatherHour>> hours({
+    required double lat,
+    required double lon,
+    required DateTime from,
+    required DateTime to,
+  }) async {
+    asked.add((lat: lat, lon: lon, from: from, to: to));
+    if (failing) {
+      throw const WeatherUnavailable();
+    }
+    return answer;
+  }
+}