Add start/end time fields to CalendarEvent (#28) (#29)

## Summary
Closes #28.

- Add optional `start_time` / `end_time` (`TimeField`) on `CalendarEvent`
- Expose times in Django admin list + form
- Replace hardcoded calendar template times with `time_range_display`
- Add migration `0013` and unit coverage for time formatting

## Test plan
- [ ] `python manage.py migrate`
- [ ] `python manage.py test schasite.tests.CalendarEventTests`
- [ ] In admin, edit an event with start/end times and confirm calendar page shows them
- [ ] Confirm events without times hide the Time rowReviewed-on: #29

schasite/migrations/0013_calendarevent_start_end_time.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.0.7 on 2026-08-04 17:59
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('schasite', '0012_alter_payments_date'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='calendarevent',
+            name='end_time',
+            field=models.TimeField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='calendarevent',
+            name='start_time',
+            field=models.TimeField(blank=True, null=True),
+        ),
+    ]