Add support for image result

Instead of a xyz service get an image back to be overlayed on the map, without recomputing results and the map pans.

This functionality should be togglable in the backend config.

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Prev title: Integrate map-based previews for synchronous workflow execution

Support `POST /results` API.

Start with the existing POST /results API and trace how synchronous workflow results reach the map. Check the backend configuration path to determine where this behavior can be toggled, then verify that an image result can be overlaid without recomputing results or panning the map.

Written by the indexing model from the issue text.