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/src/ea_chatbot/api/utils.py

@@ -1,5 +1,5 @@
 from datetime import datetime, timedelta, timezone
-from typing import Optional, Union, Any, List, Dict
+from typing import Optional, Any
 from jose import JWTError, jwt
 from pydantic import BaseModel
 from langchain_core.messages import BaseMessage