Mypy and numpy incompatible through `make install-dev`

I had the issue that make install-dev installed a numpy 2.x.x version, which was incompatible with our pinned mypy version in pyproject.toml. We should move to a lockfile approach like in LightlyTrain and clean up the dependency handling overall while we're at it.

• unpin the mypy version, and instead use >= operator
• create make lock command in the Makefile with a pinned EXCLUDE_NEWER_DATE inside the Makefile, so that the lock does not constantly change
• use uv run --frozen in all the make commands instead of python
• move dev optional dependencies to a uv group instead (dependency-groups), so that the dev group is not actually distributed via PyPI
• update the CONTRIBUTING.md file to reflect the workflow changes (no need to activate the virtual env anymore)
• update any agent files (CLAUDE.md etc.) to instruct the agent to always use uv run --frozen instead of python

If anyone would like to work on this (or parts of it; we can also split this into sub-issues), let me know!