As it's just you I'd stick with Ruby on Rails 8[1] as you already know it and I think it could realistically easily achieve what you're proposing.

There's lots of libraries for calling out external AI services. e.g. something like FastMCP[2]. From the sound of it that's all you need.

I'd use Hotwire[3] for the frontend and Hotwire Native if you want to rollout an app version quickly. I'd back it with SolidCache, SolidQueue, etc

I'd use Kamal[4] to run it on cheap hosting using on something cheap from Hetzner.

1. https://rubyonrails.org/

2. https://github.com/yjacquin/fast-mcp

3. https://hotwired.dev/

4. https://kamal-deploy.org/