Add UPC code to product_created Kafka event

Resolves #19

Added the upc field to the product_created event payload to allow
event consumers to access the UPC code without additional lookups.
Updated integration tests to validate the upc field is published.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mikesir87

test/integration/productCreation.integration.spec.js

@@ -89,6 +89,7 @@ describe("Product creation", () => {
     await kafkaConsumer.waitForMessage({
       id: createdProduct.id,
       action: "product_created",
+      upc: "100000000002",
     });
   }, 15000);