The whole YOLO-ing signature inputs and storage ops aside, something like this should never pass even cursory code review:

    catch (EntityNotFoundException) when (upsert)

So, yeah, all they're doing here is adding an 'if' that is literally 100s of times more expensive than a regular function call, which isn't great for a database-ish storage system, where you'd expect concepts like 'indexes' and 'table stats' to come into play.

Not trying to be unnecessarily dismissive or anything, but at this rate, this codebase is not going in the direction that matches the ambitions of the project website.

It would be more helpful to know which model was used, rather than “Copilot.”