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_query_analyzer.py

@@ -1,7 +1,6 @@
 import pytest
 from unittest.mock import MagicMock, patch
 from ea_chatbot.graph.nodes.query_analyzer import query_analyzer_node, QueryAnalysis
-from ea_chatbot.graph.state import AgentState
 
 @pytest.fixture
 def mock_state():