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_server/test/unit/weather/open_meteo_test.dart

@@ -0,0 +1,91 @@
+import 'package:halkyon_train_server/src/weather/open_meteo.dart';
+import 'package:test/test.dart';
+
+/// Open-Meteo's answer (`timeformat=unixtime`) for [hours] starting at
+/// [start], each hour's figures as [figures] gives them.
+Map<String, Object?> openMeteoAnswer(
+  DateTime start,
+  int hours, {
+  Object? Function(String variable, int hour)? figures,
+}) => {
+  'latitude': 52.52,
+  'longitude': 13.42,
+  'utc_offset_seconds': 0,
+  'timezone': 'GMT',
+  'hourly': {
+    'time': [
+      for (var i = 0; i < hours; i++)
+        start.add(Duration(hours: i)).millisecondsSinceEpoch ~/ 1000,
+    ],
+    for (final variable in hourlyVariables)
+      variable: [
+        for (var i = 0; i < hours; i++) (figures ?? _figures)(variable, i),
+      ],
+  },
+};
+
+Object? _figures(String variable, int hour) => switch (variable) {
+  'temperature_2m' => 12.5 + hour,
+  'apparent_temperature' => 10.1 + hour,
+  'relative_humidity_2m' => 80,
+  'precipitation' => 0.2,
+  'weather_code' => 61,
+  'wind_speed_10m' => 4.2,
+  'wind_gusts_10m' => 9.8,
+  'wind_direction_10m' => 225,
+  _ => null,
+};
+
+/// Open-Meteo's hours read (ADR 0036).
+void main() {
+  test('reads each hour with its figures, in UTC', () {
+    final start = DateTime.utc(2026, 10, 30, 7);
+    final [first, second] = hoursOf(openMeteoAnswer(start, 2));
+    expect(first.time, start);
+    expect(first.time.isUtc, isTrue);
+    expect(first.temperature, 12.5);
+    expect(first.feelsLike, 10.1);
+    expect(first.humidity, 80);
+    expect(first.precipitation, 0.2);
+    expect(first.code, 61);
+    expect(first.wind, 4.2);
+    expect(first.gusts, 9.8);
+    expect(first.windFrom, 225);
+    expect(second.time, start.add(const Duration(hours: 1)));
+    expect(second.temperature, 13.5);
+  });
+
+  test('leaves a figure out where it is missing, short or not a number', () {
+    final answer = openMeteoAnswer(
+      DateTime.utc(2026, 10, 30, 7),
+      2,
+      figures: (variable, hour) =>
+          variable == 'temperature_2m' && hour == 1 ? null : 'x',
+    );
+    (answer['hourly']! as Map<String, Object?>)['wind_speed_10m'] = [3.0];
+    final [first, second] = hoursOf(answer);
+    expect(first.temperature, isNull);
+    expect(first.wind, 3.0);
+    expect(second.wind, isNull);
+    expect(second.temperature, isNull);
+  });
+
+  test('reads no hours from an answer without times', () {
+    expect(hoursOf({}), isEmpty);
+    expect(
+      hoursOf({
+        'hourly': {'time': 'x'},
+      }),
+      isEmpty,
+    );
+    expect(
+      hoursOf({
+        'hourly': {
+          'time': ['2026-10-30T07:00'],
+        },
+      }),
+      isEmpty,
+      reason: 'only unixtime is asked for',
+    );
+  });
+}