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

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

backend/app/services/users.py

@@ -1,8 +1,8 @@
 """User management service: CRUD helpers against DuckDB."""
 from typing import Any
 
-from backend.app.db import get_conn, get_write_lock
-from backend.app.services.auth import hash_password
+from ..db import get_conn, get_write_lock
+from .auth import hash_password
 
 
 async def get_user(user_id: str) -> dict[str, Any] | None: