fix(model-install): persist remote access_token for resume after restart

[DustyShoe]

Summary

Persist and restore access_token for paused remote model installs using a minimal marker schema (source + access_token).

Related Issues / Discussions

Fixes: #8928

QA Instructions

1. Start installing a gated HuggingFace model (or any token-protected URL) with a valid access token.
2. Wait until download is partially complete, then force-stop the backend.
3. Restart the backend and click Resume on the paused install job in Model Manager queue.
4. Verify the job resumes downloading instead of failing with a missing-token error.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)