fix(payments): make checkout attempts idempotent and reconcilable

apps/payments/models.py

@@ -97,6 +97,7 @@ class Payment(BaseModel):
         related_name="payments",
         editable=False,
     )
+    idempotency_key = models.UUIDField(null=True, blank=True, editable=False)
     authority = models.CharField(max_length=64, unique=True, null=True, blank=True, editable=False)
     status = models.IntegerField(choices=OrderStatusChoices.choices, default=OrderStatusChoices.INIT, editable=False)
     ref_id = models.CharField(max_length=64, null=True, blank=True, editable=False)
@@ -104,6 +105,15 @@ class Payment(BaseModel):
     card_hash = models.CharField(max_length=128, null=True, blank=True, editable=False)
     verified_at = models.DateTimeField(null=True, blank=True, editable=False)
 
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                fields=["user", "event", "idempotency_key"],
+                condition=Q(idempotency_key__isnull=False),
+                name="payment_user_event_idempotency_key_uniq",
+            ),
+        ]
+
     def clean(self):
         if self.discount_amount and self.amount + self.discount_amount != self.base_amount:
             raise ValidationError({"amount": "amount + discount_amount must equal base_amount"})
@@ -119,4 +129,3 @@ class Payment(BaseModel):
 
     def __str__(self):
         return f"{self.user.email}:{self.event} - {self.get_status_display()}"
-