feat: derive event lifecycle status from dates and flags

Replace persisted event statuses with publication and cancellation flags, and compute public lifecycle labels from event timing and registration capacity. Keep filters, registration checks, and profile responses aligned with the derived status.

apps/events/tests/integration/test_event_changes.py

@@ -30,10 +30,10 @@ def test_event_reschedule_and_cancellation_notify_registered_users(client, user_
     assert event.location == "New hall"
     assert notify.call_args.args[1]["type"] == "event_rescheduled"
     sms.assert_called_with(attendee.mobile, "event_reschedule", event.title)
-    cancelled = _put(client, path, {"status": Event.StatusChoices.CANCELLED}, auth_headers(staff))
+    cancelled = _put(client, path, {"is_cancelled": True}, auth_headers(staff))
     assert cancelled.status_code == 200, cancelled.content
     event.refresh_from_db()
-    assert event.status == Event.StatusChoices.CANCELLED
+    assert event.is_cancelled
     assert notify.call_args.args[1]["type"] == "event_cancelled"
     assert sms.call_count == 2