Demo: Testcontainers integration tests for Shop

Step by step

1. Add Testcontainers and the PostgreSQL module dependencies (verify versions).
2. Create a shared base configuration with a static container and @ServiceConnection.
3. Write a @DataJpaTest for a custom product search query.
4. Write a @SpringBootTest that creates an order and reads it back with migrations applied.
5. Planned mistake: an H2 test passes but the same query fails on PostgreSQL because of a function difference; move it to the Testcontainers test.
6. Run the suite with Docker only, on a clean checkout.