Sync README to Docker Hub overview on publish

[Lucifix]

What does this change?

The Docker Hub repository shows "No overview available". This PR adds a peter-evans/dockerhub-description@v5 step at the end of the merge job. On each publish from main it pushes README.md as the Docker Hub overview and sets the short description to the README tagline.

• Main only (github.ref == 'refs/heads/main'), so a release tag or a manual run from another ref can't overwrite the overview with a different README.
• Only when the Docker Hub secrets are set, the same condition the mirror step uses.
• Sparse checkout of README.md: the merge job never checked out the repo before, because it only needs the image digests.
• enable-url-completion: true turns relative markdown links (docker-compose.yml, LICENSE, CONTRIBUTING.md, …) into full GitHub URLs, since Docker Hub can't resolve relative paths.
• README screenshots: the three <img src> values now use full raw.githubusercontent.com URLs. The action's URL rewriting only handles markdown syntax, not HTML <img> tags, so relative paths would show as broken images on Docker Hub. The new URLs work on GitHub too; I checked that all three load.

Reviewer notes

• Token scope: the action's docs say the Docker Hub token needs Read, Write, Delete access. If DOCKERHUB_TOKEN is Read & Write only, image mirroring still works but this step will fail with a 403. In that case, create a new token with that scope and update the secret.
• The repository name comes from the secret and repo name (DOCKERHUB_USERNAME/model-workshop-manager), the same way the mirror step builds it.
• Sizes: the README is about 12.2 KB (Docker Hub allows 25 KB) and the short description is 90 characters (limit 100).
• I parsed the workflow YAML locally. The step itself only runs after merge to main.

Related issue

Follow-up to #8 / #9.

Checklist

• Ran npm run lint:fix and npm run fmt at the repo root (fmt:check passes)
• Added/updated backend tests for behavior changes (if applicable): n/a, CI/docs only
• npm run typecheck and npm run build pass: n/a, no app code changed
• Followed existing conventions (comments explain why not what)

Screenshots

n/a