Implement debouncing for images

- A network call for the image is made every time a character is entered or deleted in the `img` tag or in the `![image]()` Markdown syntax.
- A network call for all images is made every time something is typed in the input box containing images, as the output is rendered live.
- Implement [debouncing(https://www.tutorialspoint.com/what-is-debouncing-in-javascript) to reduce network requests for images.

No file or test is named in the issue. Start by locating the live Markdown rendering and image-request paths, then verify that rapid edits make fewer image requests while the preview still updates correctly after typing pauses.

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