Show in-progress multi-day events on the calendar (#33)

## Summary
Closes #32.
- Keep an event on Upcoming until its end date is over. If `end_time` is set, it drops off once that time passes on the end date, so an in-progress multi-day event stays visible.
- Render the Past tab from `past_events` instead of `future_events`.
- Show the start–end date range when the dates differ.

## Test plan
- [ ] Multi-day event that started yesterday and ends tomorrow appears under Upcoming
- [ ] Event whose end time has already passed appears under Past
- [ ] Future single-day event still appears under Upcoming
- [ ] Past tab lists finished events only
- [ ] `uv run python manage.py test schasite.tests.CalendarEventTests`

Reviewed-on: #33

schasite/templates/schasite/calendar2.html

@@ -114,7 +114,7 @@
                                     </div>
                                     <div class="col-md-8">
                                         <div class="mb-3">
-                                            <p class="mb-1"><i class="bi bi-calendar-event text-success me-2"></i><strong>Date:</strong> {{ event.start_date }}</p>
+                                            <p class="mb-1"><i class="bi bi-calendar-event text-success me-2"></i><strong>Date:</strong> {% if event.end_date and event.end_date != event.start_date %}{{ event.start_date }} – {{ event.end_date }}{% else %}{{ event.start_date }}{% endif %}</p>
                                             {% if event.time_range_display %}
                                             <p class="mb-1"><i class="bi bi-clock text-success me-2"></i><strong>Time:</strong> {{ event.time_range_display }}</p>
                                             {% endif %}
@@ -144,7 +144,7 @@
             <div class="tab-pane fade" id="past" role="tabpanel">
                 <div class="row g-4">
                     <!-- Past Event 1 -->
-                    {% for event in future_events %}
+                    {% for event in past_events %}
                         <div class="col-lg-6">
                             <div class="card h-100 shadow-sm">
                                 <div class="card-header bg-secondary text-white">
@@ -160,7 +160,7 @@
                                         </div>
                                         <div class="col-md-8">
                                             <div class="mb-3">
-                                                <p class="mb-1"><i class="bi bi-calendar-event text-secondary me-2"></i><strong>Date:</strong> {{ event.start_date }}</p>
+                                                <p class="mb-1"><i class="bi bi-calendar-event text-secondary me-2"></i><strong>Date:</strong> {% if event.end_date and event.end_date != event.start_date %}{{ event.start_date }} – {{ event.end_date }}{% else %}{{ event.start_date }}{% endif %}</p>
                                                 {% if event.time_range_display %}
                                                 <p class="mb-1"><i class="bi bi-clock text-secondary me-2"></i><strong>Time:</strong> {{ event.time_range_display }}</p>
                                                 {% endif %}