feat: snapshot payment provider keys on orders

backend/migrations/112_add_payment_order_provider_key_snapshot.sql

@@ -0,0 +1,10 @@
+ALTER TABLE payment_orders ADD COLUMN provider_key VARCHAR(30);
+
+UPDATE payment_orders
+SET provider_key = (
+    SELECT provider_key
+    FROM payment_provider_instances
+    WHERE CAST(id AS TEXT) = payment_orders.provider_instance_id
+)
+WHERE provider_key IS NULL
+  AND provider_instance_id IS NOT NULL;