Use `uv run` in local development and Docker

Why are we doing this?

Whilst switching dependency management tooling over to uv, a some potential updates to justfile recipes were noted in a (PR review comment). These would be nice improvements to simplify the recipes we have and be more consistent across projects.

How will we know when it's done?

• Local developer commands (e.g. tests, linting, formatting) are run via uv run.
• Production and CI commands use uv run.
• we've removed $BIN and $VIRTUAL_ENV from the justfile.

What are we doing?

Justfile updates:

• diff with the latest repo template justfile and include any improvements from there
• use uv run … instead of $BIN/python …,
• Investigate whether VIRTUAL_ENV and $BIN can be removed from recipes/configuration; remove it if possible.
• Investigate alternatives to hard-coding the python version in the bump-cut-off recipe (see discussion here)
• Review the justfile for any further improvements or simplifications related to the move to uv

Dockerfile updates:

• diff with the latest repo template Dockerfile and include any improvements from there
• Update Dockerfile to run commands via uv run
• Verify local, CI, and production workflows continue to behave as expected

Defining delivery tasks guidance

Start by diffing the Justfile and Dockerfile against the linked repo-template versions, then review the existing recipes, especially bump-cut-off. Update the local, CI, and production command paths to use uv run where appropriate, investigate removing $BIN and VIRTUAL_ENV, and verify the workflows continue to behave as expected.