feat(events): harden registration checkout lifecycle

apps/events/models.py

@@ -108,8 +108,15 @@ class Event(BaseModel):
 
     @property
     def current_attendees_count(self):
-        """Count confirmed attendees"""
-        return self.registrations.filter(status__in=[Registration.StatusChoices.CONFIRMED, Registration.StatusChoices.ATTENDED], is_deleted=False).count()
+        """Count confirmed attendees and currently held paid-checkout seats."""
+        now = timezone.now()
+        return self.registrations.filter(is_deleted=False).filter(
+            models.Q(status__in=[Registration.StatusChoices.CONFIRMED, Registration.StatusChoices.ATTENDED])
+            | models.Q(
+                status=Registration.StatusChoices.PENDING,
+                checkout_expires_at__gt=now,
+            )
+        ).count()
 
     @property
     def has_available_slots(self):
@@ -132,9 +139,24 @@ class Registration(BaseModel):
         SYSTEM = 'system', 'System'
         HISTORICAL = 'historical', 'Historical backfill'
 
+    class CancellationReasonChoices(models.TextChoices):
+        CHECKOUT_EXPIRED = 'checkout_expired', 'Checkout expired'
+        USER_CANCELLED = 'user_cancelled', 'User cancelled checkout'
+        EVENT_CANCELLED = 'event_cancelled', 'Event cancelled'
+        PAYMENT_FAILED = 'payment_failed', 'Payment failed'
+
     event = models.ForeignKey(Event, on_delete=models.CASCADE, related_name='registrations')
     user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='event_registrations')
     registered_at = models.DateTimeField(auto_now_add=True)
+    checkout_started_at = models.DateTimeField(null=True, blank=True, editable=False)
+    checkout_expires_at = models.DateTimeField(null=True, blank=True, editable=False)
+    cancellation_reason = models.CharField(
+        max_length=32,
+        choices=CancellationReasonChoices.choices,
+        blank=True,
+        default='',
+        editable=False,
+    )
     status = models.CharField(max_length=10, choices=StatusChoices.choices,
                               default=StatusChoices.PENDING)
     attendance_verified_at = models.DateTimeField(null=True, blank=True)