Fix zap receipt not being stored correctly

fixes #194

app/controllers/webhooks_controller.rb

@@ -13,7 +13,7 @@ class WebhooksController < ApplicationController
         paid_at: Time.parse(@payload[:settled_at]).to_i,
         preimage: @payload[:preimage]
       )
-      @zap.update! receipt: @zap_receipt.to_h
+      @zap.update! receipt: zap_receipt.to_h
       NostrManager::PublishZapReceipt.call(zap: @zap)
     end