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/checkout.py

@@ -80,7 +80,7 @@ def expire_stale_registration_checkouts(*, now=None, event_id=None):
                 continue
             reason = (
                 Registration.CancellationReasonChoices.EVENT_CANCELLED
-                if registration.event.status == Event.StatusChoices.CANCELLED
+                if registration.event.is_cancelled
                 else Registration.CancellationReasonChoices.CHECKOUT_EXPIRED
             )
             if mark_checkout_cancelled(registration, reason):
@@ -105,7 +105,7 @@ def expire_user_registration_checkouts(*, event, user, now):
     for registration in registrations:
         reason = (
             Registration.CancellationReasonChoices.EVENT_CANCELLED
-            if event.status == Event.StatusChoices.CANCELLED
+            if event.is_cancelled
             else Registration.CancellationReasonChoices.CHECKOUT_EXPIRED
         )
         if mark_checkout_cancelled(registration, reason):