Spring transaction boundary experiments

Requires Java 17 and Gradle 8.14.2.
Run from the extracted directory: gradle -p spring-transactions run
Gradle downloads pinned dependencies from Maven Central.
Uses an isolated in-memory H2 database and synthetic account data.
Expected: ALL 5 SCENARIOS PASSED
Checks rollback after one and after both SQL updates, self invocation, connection sharing/return, and JPA flush/rollback.
This is learning code, not a production transfer service.
