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_database_inspection.py

@@ -1,7 +1,7 @@
 import pytest
 import pandas as pd
 import os
-from unittest.mock import MagicMock, patch
+from unittest.mock import MagicMock
 from ea_chatbot.utils.database_inspection import get_primary_key, inspect_db_table, get_data_summary
 
 @pytest.fixture