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/api/test_persistence.py

@@ -3,10 +3,8 @@ from fastapi.testclient import TestClient
 from unittest.mock import MagicMock, patch, AsyncMock
 from ea_chatbot.api.main import app
 from ea_chatbot.api.dependencies import get_current_user
-from ea_chatbot.history.models import User, Conversation, Message, Plot
+from ea_chatbot.history.models import User, Conversation
 from ea_chatbot.api.utils import create_access_token
-from datetime import datetime, timezone
-import json
 
 client = TestClient(app)