Add automated tests for CRUD operations with pytest

[prokhin]

Currently, the project does not have automated tests for the CRUD operations. To improve reliability and ensure future changes do not break core functionality, we need to implement automated tests.

Tasks:

Add pytest as a testing framework.

Write tests that cover the main CRUD operations (Create, Read, Update, Delete).

Ensure tests can be executed both locally and inside a container.

Integrate tests into the existing Docker Compose setup (e.g., add a test service in docker-compose.yml).

Update project documentation with instructions on how to run the tests.

Acceptance Criteria:

All CRUD operations are covered by tests.

Tests run successfully via Docker Compose.

Documentation is updated with test execution steps.