Parallel integration test execution using PostgreSQL database/schema isolation

Summary

Integration tests currently run sequentially. With 50+ test classes, this results in long feedback cycles.

Problem

Each test class uses TRUNCATE CASCADE for isolation before execution, which prevents parallel runs — concurrent tests would interfere with each other's data in the shared public schema.

On modern multi-core CPUs, sequential test execution leaves most cores idle, failing to utilise available hardware capacity.

Goal

Enable integration test classes to run in parallel without data interference, reducing overall test suite execution time.

Context

• Spring Boot integration tests (@SpringBootTest)
• All test classes share a single Spring application context
• PostgreSQL
• Flyway for schema migrations
• ~50+ integration test classes, each setting up and tearing down data via JDBC

Start by examining the Spring Boot integration test setup, JDBC isolation, shared application context, and Flyway migration flow described in the issue. The work is done when the 50+ integration test classes can run in parallel with isolated PostgreSQL databases or schemas, without data interference, while preserving test correctness and reducing execution time.