feat(config): add support for separate test database via .env.test

[EmiFunes91]

Summary

This PR introduces support for using a separate PostgreSQL database during testing by adding a new .env.test file and dynamic configuration logic.

Changes

• Adds .env.test to define ENVIRONMENT=test and POSTGRES_DB=app_test
• Updates config.py to automatically switch the database based on environment
• Documents usage in README.md under a new Testing Environment section

Motivation

Currently, the development and testing environments use the same database (app), which can lead to data collisions or accidental data loss.

This PR addresses Issue #125 by separating configuration and runtime behavior cleanly, without introducing breaking changes.

Notes

• Docker Compose is reused for both dev and test; no new service is introduced
• This is backwards compatible and does not interfere with existing setups
• CI environments can now easily isolate their test database

Let me know if you'd like .env.test to be excluded via .gitignore or if you'd prefer a docker-compose.override.yml for full isolation.

[EmiFunes91]

Hi maintainers!

The check-labels workflow is currently failing because it requires one of the following labels:
breaking, security, feature, bug, refactor, upgrade, docs, lang-all, or internal.

Could you please add one of them? I believe feature would be appropriate in this case.

Thank you very much!