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/output/01-two-databases.txt

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+# Two databases, each with its own DataSource, Flyway, EntityManagerFactory and transaction manager
+
+
+--- the two DataSources (one pool each, configured separately) ---
+customersDataSource  pool=customers-pool  max=4  url=jdbc:h2:mem:customers  primary=true
+ordersDataSource     pool=orders-pool     max=8  url=jdbc:h2:mem:orders  primary=false
+
+--- Flyway ran once per database, each with its own history ---
+customers flyway_schema_history: [1 customer]
+orders    flyway_schema_history: [1 purchase order, 2 index customer]
+
+--- what each EntityManagerFactory manages ---
+EntityManagerFactory beans: [customersEntityManagerFactory, ordersEntityManagerFactory]
+customersEntityManagerFactory    entities=[Customer]
+ordersEntityManagerFactory       entities=[PurchaseOrder]
+
+--- transaction managers ---
+beans: [customersTransactionManager, ordersTransactionManager]
+customersTransactionManager    primary=true
+ordersTransactionManager       primary=false
+
+--- save one customer and one order through the two repositories ---
+customers database tables: [CUSTOMER, flyway_schema_history]
+orders    database tables: [PURCHASE_ORDER, flyway_schema_history]
+rows: customers.customer=1  orders.purchase_order=1