Add OpenWeatherMap weather forecasts. Closes #48

Show 8-day Bristol home weather on the index and weekends pages.
Show destination weather per day on the trip list and trip page.
Cache forecasts in ~/lib/data/weather/ and refresh via update.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agenda/weather.py

@@ -0,0 +1,83 @@
+"""Weather forecast using OpenWeatherMap One Call API."""
+
+import json
+import os
+from datetime import datetime
+
+import pyowm
+
+
+def _cache_path(data_dir: str, lat: float, lon: float) -> str:
+    """Path for weather cache file."""
+    weather_dir = os.path.join(data_dir, "weather")
+    os.makedirs(weather_dir, exist_ok=True)
+    return os.path.join(weather_dir, f"{lat:.2f}_{lon:.2f}.json")
+
+
+def _is_fresh(path: str, max_age_hours: int = 6) -> bool:
+    """Return True if the cache file exists and is recent enough."""
+    if not os.path.exists(path):
+        return False
+    age = datetime.now().timestamp() - os.path.getmtime(path)
+    return age < max_age_hours * 3600
+
+
+def get_forecast(data_dir: str, api_key: str, lat: float, lon: float) -> list[dict]:
+    """Return 8-day daily forecast for lat/lon, caching results for 6 hours."""
+    cache_file = _cache_path(data_dir, lat, lon)
+
+    if _is_fresh(cache_file):
+        with open(cache_file) as f:
+            return json.load(f)  # type: ignore[no-any-return]
+
+    owm = pyowm.OWM(api_key)
+    mgr = owm.weather_manager()
+    result = mgr.one_call(lat=lat, lon=lon)
+
+    forecasts = []
+    for day in result.forecast_daily:
+        dt = datetime.fromtimestamp(day.ref_time)
+        temp = day.temperature("celsius")
+        forecasts.append(
+            {
+                "date": dt.date().isoformat(),
+                "status": day.status,
+                "detailed_status": day.detailed_status,
+                "temp_min": round(temp["min"]),
+                "temp_max": round(temp["max"]),
+                "precipitation_probability": day.precipitation_probability,
+                "icon": day.weather_icon_name,
+            }
+        )
+
+    with open(cache_file, "w") as f:
+        json.dump(forecasts, f)
+
+    return forecasts
+
+
+def trip_latlon(trip: object) -> tuple[float, float] | None:
+    """Return (lat, lon) for the primary destination of a trip, or None."""
+    from agenda.types import Trip
+
+    assert isinstance(trip, Trip)
+    for item in list(trip.accommodation) + list(trip.conferences):
+        if "latitude" in item and "longitude" in item:
+            return (float(item["latitude"]), float(item["longitude"]))
+    return None
+
+
+def get_trip_weather(data_dir: str, api_key: str, trip: object) -> dict[str, dict]:
+    """Return forecast for a trip keyed by date ISO string.
+
+    Returns an empty dict if no location is known or the API call fails.
+    """
+    latlon = trip_latlon(trip)
+    if not latlon:
+        return {}
+    lat, lon = latlon
+    try:
+        forecasts = get_forecast(data_dir, api_key, lat, lon)
+    except Exception:
+        return {}
+    return {f["date"]: f for f in forecasts}