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_logging_e2e.py

@@ -1,8 +1,7 @@
-import os
 import json
 import pytest
 import logging
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
 from ea_chatbot.graph.workflow import app
 from ea_chatbot.graph.state import AgentState
 from ea_chatbot.utils.logging import get_logger