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/graph/test_checkpoint.py

@@ -30,7 +30,6 @@ async def test_get_checkpointer_initialization():
         checkpoint.get_pool = mock_get_pool
         
         # Patch AsyncPostgresSaver where it's imported in checkpoint.py
-        import langgraph.checkpoint.postgres.aio as pg_aio
         original_saver = checkpoint.AsyncPostgresSaver
         checkpoint.AsyncPostgresSaver = mock_saver_class