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_users.py

@@ -4,8 +4,8 @@ import pytest
 import pytest_asyncio
 from httpx import AsyncClient, ASGITransport
 
-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
 
 os.environ.setdefault("JWT_SECRET", "test-secret-key-for-testing-only")