Add poster image to video

[jo-elimu]

Description

• Bump model to 2.0.98: https://github.com/elimu-ai/model/releases/tag/model-2.0.98
• Add Image posterImage; to Video.
• Add drop-down in UI.
• Update REST API endpoint.

Why?

To display an image before the video starts playing.

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• Depends on Add poster image to video model#341

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