Add workflow to run uv lock

[browniebroke]

Description

Build on top of https://github.com/cookiecutter/cookiecutter-django/pull/5356 to add a uv lock workflow

Checklist:

• [x] I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• [x] I've updated the documentation or confirm that my change doesn't require any updates

Rationale

At time of writing, Dependabot doesn't yet support uv.lock file: https://github.com/dependabot/dependabot-core/issues/10478 but it does support package deps in pyproject.toml, which means that we will get PRs to updates our template deps, but they will leave the lockfile out of sync.

This workflow is a workaround for that, and will attempt to regenerate the lockfile each time the pyproject.toml file is changed by a pull request.

[foarsitter]

Adding a dependency through uv add also updates uv.lock. So the lock file is kept in sync.

If we commit 'uv.lock', is a workflow like this then required?

[browniebroke]

This workflow is mainly for dependabot PRs, which AFAIK doesn't support uv.lock yet. Since it does support deps in pyproject.toml, as it's an earlier standard, PRs would leave the lock file out of date.

[foarsitter]

Clever!

[browniebroke]

Ok, let's try this