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/payments/tests/integration/test_payment_reconciliation.py

@@ -243,8 +243,8 @@ def test_late_success_after_user_or_event_cancellation_is_refunded(
     registration.cancellation_reason = reason
     registration.save(update_fields=["cancellation_reason"])
     if cancel_event:
-        payment.event.status = payment.event.StatusChoices.CANCELLED
-        payment.event.save(update_fields=["status"])
+        payment.event.is_cancelled = True
+        payment.event.save(update_fields=["is_cancelled"])
     mocker.patch("apps.payments.tasks.refund_payment.apply_async")
     paid_at = (registration.checkout_expires_at - timedelta(seconds=1)).isoformat()