Manage curated images from the admin page

### Task Summary

Curated images can be managed over the API, but only by someone comfortable with curl. Administrators need a page.

A page under Admin → CU Images. Registering takes a name people will recognise and the image reference.

The row shows PENDING → VALIDATING → READY, or FAILED. The list polls while anything is still being checked and stops when nothing is. A Log button shows the check's output, which is where a rejection explains itself — a tag that does not exist, a private image, an image that is not a computing-unit image — and what to do instead.

Each row also offers Refresh, which re-checks the same reference and picks up a moved tag, and Remove.

Done when an administrator can do all of that without touching the API, and a rejected image's reason is readable on the page.

Part of #8466

### Task Type

- [ ] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [ ] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other