Drive search form dropdowns from VALID_MIN/MAX_CONNECTIONS; warn on short transfers

Index page connection time dropdowns now iterate over valid_min_connections
and valid_max_connections passed from the view, so any change to the sets
in app.py is reflected automatically (also adds the missing 45 min option).

Add ⚠️ next to transfer times under 80 minutes in the results table;
store connection_minutes in each trip dict to support the comparison.

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

templates/index.html

@@ -47,7 +47,7 @@
         Minimum connection time (Paddington → St Pancras)
       </label>
       <select id="min_connection" name="min_connection" class="form-control">
-        {% for mins in [50, 60, 70, 80, 90, 100, 110, 120] %}
+        {% for mins in valid_min_connections %}
           <option value="{{ mins }}" {% if mins == 50 %}selected{% endif %}>{{ mins }} min</option>
         {% endfor %}
       </select>
@@ -58,7 +58,7 @@
         Maximum connection time (Paddington &rarr; St&nbsp;Pancras)
       </label>
       <select id="max_connection" name="max_connection" class="form-control">
-        {% for mins in [60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180] %}
+        {% for mins in valid_max_connections %}
           <option value="{{ mins }}" {% if mins == 110 %}selected{% endif %}>{{ mins }} min</option>
         {% endfor %}
       </select>

templates/results.html

@@ -144,7 +144,7 @@
           <br><span style="font-size:0.75rem;color:#718096">{{ row.ticket_name }}</span>
         </td>
         <td class="col-transfer" style="padding:0.6rem 0.8rem;color:#4a5568">
-          {{ row.connection_duration }}
+          {{ row.connection_duration }}{% if row.connection_minutes < 80 %} ⚠️{% endif %}
         </td>
         <td style="padding:0.6rem 0.8rem;font-weight:600">
           {{ row.depart_st_pancras }}