fix(payments): make checkout attempts idempotent and reconcilable

apps/payments/migrations/0004_payment_idempotency_key_and_more.py

@@ -0,0 +1,25 @@
+# Generated by Django 5.2.5 on 2026-09-30 17:06
+
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('events', '0015_registration_attendance_verification_source_and_more'),
+        ('payments', '0003_payment_registration'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='payment',
+            name='idempotency_key',
+            field=models.UUIDField(blank=True, editable=False, null=True),
+        ),
+        migrations.AddConstraint(
+            model_name='payment',
+            constraint=models.UniqueConstraint(condition=models.Q(('idempotency_key__isnull', False)), fields=('user', 'event', 'idempotency_key'), name='payment_user_event_idempotency_key_uniq'),
+        ),
+    ]