Setup Integration Testing with Testcontainers

[viceda-s]

Problem: Mocks don't validate SQL correctness, transaction behavior, or constraint violations.

Acceptance Criteria:

• Integrate PostgreSQL Testcontainers in the test suite.
• Write at least one end-to-end integration test (testing full DB persistence logic without mocks) for the WorkoutService.

[viceda-s]

Integration testing infrastructure with PostgreSQL Testcontainers has been successfully established. The test suite now runs against a real database, verifying complete persistence flows without mocks. Closing issue.