Add multi-datasource module: two databases with per-database DataSource, Flyway, EntityManagerFactory and transaction manager on Boot 4.1

Working setup plus one small application per way of getting it wrong (no @Primary, @Primary only, url vs jdbc-url, wrong repository package, Flyway auto-configuration), plain vs named @Transactional, and ChainedTransactionManager with a failing commit. Transcripts are written by the tests.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Uu7q8vPeREyT4218EJPzz1

multi-datasource/scripts/facts/RepeatedEnable.java

@@ -0,0 +1,9 @@
+import org.springframework.context.annotation.Configuration;
+import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
+
+// Two scans on ONE class, the natural first attempt. Does not compile: the annotation is not repeatable.
+@Configuration
+@EnableJpaRepositories(basePackages = "a.customers", entityManagerFactoryRef = "customersEntityManagerFactory")
+@EnableJpaRepositories(basePackages = "a.orders", entityManagerFactoryRef = "ordersEntityManagerFactory")
+class RepeatedEnable {
+}