Implement payment backends and interaction

Advances #3 (closed)

aleksis/apps/tezor/models/invoice.py

@@ -56,7 +56,7 @@ class Invoice(BasePayment, PureDjangoModel):
 
     class Meta:
         constraints = [
-            models.UniqueConstraint(fields=["transaction_id", "group"], name="number_uniq_per_group")
+            models.UniqueConstraint(fields=["number", "group"], name="number_uniq_per_group")
         ]
 
     @property