OdbBackend: acquire the GIL in the callbacks, fix write_cb's type argument - #1496

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: codespell-project/codespell-problem-matcher@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/