Switch to the package manager uv

We should update the app to use UV to match buildpack best practice recommendations

```
remote: ! Note: We recently added support for the package manager uv:
remote: ! https://devcenter.heroku.com/changelog-items/3238
remote: !
remote: ! It's now our recommended Python package manager, since it
remote: ! supports lockfiles, is faster, gives more helpful error
remote: ! messages, and is actively maintained by a full-time team.
remote: !
remote: ! If you haven't tried it yet, we suggest you take a look!
```