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/models.py

@@ -77,17 +77,32 @@ class CalendarEvent(TimeInfoBase):
     def has_date(self):
         return not self.start_date is None
 
+    def _has_ended(self, now=None):
+        """True once the event is over. In-progress multi-day events stay open
+        through end_date, and through end_time on that date when one is set.
+        """
+        if not self.has_date():
+            return None
+        if now is None:
+            now = datetime.datetime.now()
+
+        end_date = self.end_date or self.start_date
+        if self.end_time:
+            return now >= datetime.datetime.combine(end_date, self.end_time)
+        return now.date() > end_date
+
     def past_event(self):
-        if self.has_date():
-            return self.start_date <= datetime.datetime.now().date()
-        else:
+        ended = self._has_ended()
+        if ended is None:
             return False
+        return ended
 
     def future_event(self):
-        if self.has_date():
-            return self.start_date > datetime.datetime.now().date()
-        else:
+        """Upcoming or currently in progress."""
+        ended = self._has_ended()
+        if ended is None:
             return False
+        return not ended
 
     def no_date(self):
         return not self.has_date()