[WIP] Project structure improvements

[SnakeOilSalesman]

Would like to help to improve multiple unexpected in Python project moments.

This particular PR motivation:

1. gitignore has some duplicates which is not dramatically bad in general but could be improved
2. modern Python projects usually provide "tools" settings via pyproject.toml file
3. uv.lock file could be added and used instead of outdated requriements.txt approach, which will be made in a following PR alongside with more complex changes
4. setup.cfg file is empty and is obsolete. Since it is not used anywhere, it is safe to drop it
5. better split main entry point into two modules: one where "app" instance is created and configured; and second where web-server is actually started. Such approach is more flexible. Uvicorn might be replaced in the future, approach with pydantic-settings parsing runner arguments might be avoided, etc.