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>

app.py

@@ -36,7 +36,13 @@ DESTINATIONS = {
 @app.route('/')
 def index():
     today = date.today().isoformat()
-    return render_template('index.html', destinations=DESTINATIONS, today=today)
+    return render_template(
+        'index.html',
+        destinations=DESTINATIONS,
+        today=today,
+        valid_min_connections=sorted(VALID_MIN_CONNECTIONS),
+        valid_max_connections=sorted(VALID_MAX_CONNECTIONS),
+    )
 
 
 VALID_MIN_CONNECTIONS = {45, 50, 60, 70, 80, 90, 100, 110, 120}