Implement Pytest automation tests and GitHub workflow

[karastoyanov]

📝 Description

This PR adds automated testing for the backend:
• Adds pytest setup with fixtures for Flask app and in-memory SQLite database.
• Creates dummy JSON test data for /register route scenarios (valid registration, weak passwords, missing fields, duplicate usernames).
• Adds Pytest tests covering registration functionality to ensure backend correctness.
• Implements a GitHub Actions workflow that runs all backend tests on every pull request to master, enforcing CI/CD best practices.

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🔗 Linked Issue (Required)

Closes: #13

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🧪 Testing

• Verified that all tests run successfully locally using pytest.

pytest -v # Needs to be executed from backend/ directory

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🎓 Learning Reflection

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📸 Screenshots / Logs (if applicable)

platform darwin -- Python 3.9.6, pytest-8.4.2, pluggy-1.6.0 -- <path-to-local-venv-python3>
cachedir: .pytest_cache
rootdir: <local-path-to-rootdir>
plugins: flask-1.3.0
collected 4 items

tests/test_auth_register.py::test_register_success PASSED                                                                                                                    [ 25%]
tests/test_auth_register.py::test_register_weak_password PASSED                                                                                                              [ 50%]
tests/test_auth_register.py::test_register_missing_fields PASSED                                                                                                             [ 75%]
tests/test_auth_register.py::test_register_duplicate_username PASSED                                                                                                         [100%]

================================================================================ 4 passed in 0.76s =================================================================================