I recently watched a talk[1] on this exact use case: a RAG system that runs on localhost with a simple web UI, and based on very powerful text processing, and a simple backend (PHP and sqlite3 with FTS and vector search extensions).

You can see the project page here: https://textualization.com/ragged/

src and scripts here: https://github.com/Textualization/the-ragged-edge-box

[1] video presentation about the project https://www.youtube.com/watch?v=_fJFuL2pLvw