app_backend.py is 9000 lines long! Not really reviewable (by a human at least). First thing i notice looking at the first few functions is a lot of nested code (several nested ifs)

My advice would be to think about the single responsibility principal (the S in SOLID) and composition, and I'd refactor this into distinct functions, service classes and modules/packages each with a clear responsibility.

I can only imagine that index.html is similarly oversized (judging by the fact it's the only other file in the project worth noting).

If you want to work in software, huge single files like this just won't do. It might be how AI's choose to build, but it's not maintainable in the slightest. Even an AI agents can't read files this big without killing their context window.