Refactor import statements across multiple modules to use relative paths for better organization and maintainability.

Co-authored-by: Copilot <copilot@github.com>

backend/tests/test_auth.py

@@ -1,6 +1,6 @@
 """Integration tests for auth endpoints using in-memory DuckDB."""
-from backend.app.main import app
-from backend.app import db as db_module
+from app.main import app
+from app import db as db_module
 from httpx import AsyncClient, ASGITransport
 import os
 import pytest