Add versions.json release asset

[andriiryzhkov]

• Add dtai versions CLI command that generates output/versions.json from model.yaml files, mapping each non-skipped model ID to its version string
• Update build workflow to generate and upload versions.json as a release asset alongside .dtmodel files
• darktable can fetch versions.json from the latest release to check for model updates without downloading full model packages

[andriiryzhkov]

@TurboGit : Can you check this?

[TurboGit]

Looks good, thanks!