ci: switch (partly) to uv?

Following up on #1470, I would like to propose using uv and the setup-uv action in ci.

I would like to discuss.

Easy

1. uv tree can replace pipdeptree

Decision needed

1. can uv sync --all-extras be used in the ci step that tests installation of all dependency groups? I would then not use the pre-built wheel of rdmo from the previous step.
2. maybe the pre-built wheel is most important for the end-to-end tests, where testing backend plus compiled js frontend is tested?

Open questions

1. should uv be used even more?
2. add a lockfile for devs and ci runs? this could pin the versions of dependencies while working on a release, no more arguments over i used this version vs you used another
3. but this lockfile would not be used in production
4. should we switch to uv build backend vs setuptools?

I am in favor of the first three suggestions. What do you think?

Start by inspecting the existing CI steps that use pipdeptree, install dependency groups, build the rdmo wheel, and run end-to-end tests. Compare those workflows with uv and setup-uv, then resolve the listed questions about sync, lockfiles, and the build backend; done means the agreed CI changes work consistently without weakening installation or end-to-end coverage.