Add single image scan functionality to Web UI

### Context

Currently, users can scan a single image using the bai CLI manager, but this functionality is not available in the Web UI. Even admins cannot perform a single image scan through the Web UI.

### Acceptance criteria

- Users should be able to scan a single image by specifying its name directly in the Web UI.
- The functionality should mirror the existing CLI command: `./backend.ai mgr image rescan "nvcr.io/nvidia/pytorch:25.05-py3"`.

### Other information

This feature is requested to enhance the Web UI capabilities and provide parity with the CLI tool.

[View original message posted by Junbum Lee on 2/5/2026, 4:11:42 PM](https://teams.microsoft.com/l/message/19:77773a3171f04070acd8b1098be60045@thread.skype/1770275502505?parentMessageId=1770258119073)

JIRA Issue: FR-2027