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/history/manager.py

@@ -1,7 +1,7 @@
 from contextlib import contextmanager
 from typing import Optional, List
-from sqlalchemy import create_engine, select, delete
-from sqlalchemy.orm import sessionmaker, Session
+from sqlalchemy import create_engine, select
+from sqlalchemy.orm import sessionmaker
 from argon2 import PasswordHasher
 from argon2.exceptions import VerifyMismatchError