Add backend API validation tests

[MyouzzZ]

Summary

• add a pytest/FastAPI TestClient suite for the root health endpoint, generate-cards request validation, and quiz answer validation
• force offline mode and monkeypatch AI card generation in tests so no real Gemini API key is required
• document the backend test command and add test dependencies to backend requirements

Closes #9.
Closes #10.
Closes #11.

Validation

• .\.venv\Scripts\python.exe -m pytest tests\test_backend_api.py -q (11 passed, 1 PyPDF2 deprecation warning)
• git diff --check -- README.md backend/requirements.txt tests/test_backend_api.py