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>

templates/trip_page.html

@@ -377,6 +377,30 @@
       </div>
     {% endfor %}
 
+    {% if trip_weather %}
+    <div class="mt-3">
+    <h4>Weather forecast</h4>
+      {% set ns = namespace(has_rows=false) %}
+      <table class="table table-hover w-auto">
+      {% for day, elements in trip.elements_grouped_by_day() %}
+        {% set weather = trip_weather.get(day.isoformat()) %}
+        {% if weather %}
+        {% set ns.has_rows = true %}
+        <tr>
+          <td class="text-end text-nowrap">{{ display_date(day) }}</td>
+          <td><img src="https://openweathermap.org/img/wn/{{ weather.icon }}.png" alt="{{ weather.status }}" title="{{ weather.detailed_status }}" width="25" height="25"></td>
+          <td>{{ weather.temp_min }}–{{ weather.temp_max }}°C</td>
+          <td>{{ weather.detailed_status }}</td>
+        </tr>
+        {% endif %}
+      {% endfor %}
+      </table>
+      {% if not ns.has_rows %}
+        <p class="text-muted">Forecast not yet available (available up to 8 days ahead).</p>
+      {% endif %}
+    </div>
+    {% endif %}
+
     <div class="mt-3">
     <h4>Holidays</h4>
     {% if holidays %}