Add outbox module: Transactional Outbox with the Spring Modulith Event Publication Registry

outbox/output/01-registry-safety-net.txt

@@ -0,0 +1,20 @@
+$ mvn -pl outbox -am test -Dtest=RegistrySafetyNetTest
+(real embedded Kafka broker via @EmbeddedKafka; BillingManagement.issueInvoice()
+ publishes InvoiceIssued, @Externalized("invoices::#{#this.invoiceId}") routes it
+ through spring-modulith-events-kafka)
+
+Kafka record received -- topic=invoices key=1 value={"invoiceId":1,"sku":"GADGET-7","amountCents":4500}
+EVENT_PUBLICATION_ARCHIVE rows for InvoiceIssued: 2
+EVENT_PUBLICATION rows still outstanding for InvoiceIssued: 0
+
+[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
+
+The message really arrived on the real topic, with the real routing key (the invoice's
+own id, exactly as the SpEL expression after "::" in @Externalized specifies) and the
+real JSON payload. Two archive rows for InvoiceIssued, not one, because this module's
+test sources also declare FlakyAuditListener (see output/02) -- it is a second,
+independent listener on the same event, and with completion-mode=ARCHIVE configured in
+application.properties, both listeners' completed publications moved out of
+EVENT_PUBLICATION and into EVENT_PUBLICATION_ARCHIVE once they succeeded. Zero rows
+left outstanding in EVENT_PUBLICATION for this event type confirms neither listener is
+still pending.