feat(auth): Complete OIDC security refactor and modernize test suite

- Refactored OIDC flow to implement PKCE, state/nonce validation, and BFF pattern.
- Centralized configuration in Settings class (DEV_MODE, FRONTEND_URL, OIDC_REDIRECT_URI).
- Updated auth routers to use conditional secure cookie flags based on DEV_MODE.
- Modernized and cleaned up test suite by removing legacy Streamlit tests.
- Fixed linting errors and unused imports across the backend.

backend/tests/test_config.py

@@ -1,5 +1,3 @@
-import pytest
-from pydantic import ValidationError
 from ea_chatbot.config import Settings, LLMConfig
 
 def test_default_settings():