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/event_list.html

@@ -70,6 +70,19 @@
     Sunset: {{ sunset.strftime("%H:%M:%S") }}</li>
     </ul>
 
+  {% if home_weather %}
+  <div class="mb-2">
+    <strong>Bristol weather:</strong>
+    {% for day in home_weather %}
+      <span class="me-3 text-nowrap">
+        <small class="text-muted">{{ day.date_obj.strftime("%-d %b") }}</small>
+        <img src="https://openweathermap.org/img/wn/{{ day.icon }}.png" alt="{{ day.status }}" title="{{ day.detailed_status }}" width="25" height="25">
+        {{ day.temp_min }}–{{ day.temp_max }}°C
+      </span>
+    {% endfor %}
+  </div>
+  {% endif %}
+
   {% if errors %}
     {% for error in errors %}
       <div class="alert alert-danger" role="alert">