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_state.py

@@ -1,8 +1,5 @@
-import pytest
-from typing import get_type_hints, List
-from langchain_core.messages import BaseMessage, HumanMessage
+from typing import get_type_hints
 from ea_chatbot.graph.state import AgentState
-import operator
 
 def test_agent_state_structure():
     """Verify that AgentState has the required fields and types."""