Thumbnails in backend

[Arnei]

THIS PR WILL BREAK THUMBNAIL GENERATION WITHOUT THE RELATED BACKEND PR: opencast/opencast#7698

Fixes #814.

Currently when generating a thumbnail, we create it here in the frontend and then send it to the backend. With this patch, we instead send the time stamp it was generated from to the backend.

The goal is to let the backend generate the actual thumbnail, instead of the frontend. This avoids quality issues when generating thumbnails from low resolution videos in the frontend. It also allows for respecting institution specific thumbnail settings (e.g. encoding profiles).

The downside is that this relies on workflow properties. As such, admins will have to adapt their workflows or thumbnail generation will be broken for
them.

This change should not impact the user experience of using the editor frontend.

How to test this

Requires changes in the backend. Can otherwise be tested as is.

[github-actions]

This pull request is deployed at test.editor.opencast.org/1708/2026-06-03_15-05-20/ .
It might take a few minutes for it to become available.

[snoesberger]

Thanks for your work! I tested the PR together with the corresponding backend PR opencast/opencast#7698 locally in different scenarios and found some issues:

• direct publish Presenter/Presentation/Dual stream events -> thumbnail resolution 1280 x 720 ✅
• Presenter video with Editor generated thumbnail -> thumbnail resolution 1280 x 720 ✅
• Presentation video with Editor generated thumbnail -> thumbnail resolution 1280 x 720 ✅
• Dual stream with Editor generated Presenter thumbnail -> thumbnail resolution 1280 x 720 ✅
• Dual stream with Editor generated Presentation thumbnail "use for all tracks" -> thumbnail resolution 640 x 360 ❌
• Changing the Presenter or Presentation video generated thumbnail for a published event afterwards with a newly generated thumbnail ✅
• Discard Presenter or Presentation video generated thumbnail for a published event afterwards -> nothing happens if I press the button "Discard" ❌
• Upload a new thumbnail image for a published Presenter, Presentation or Dual stream event which already has a generated thumbnail -> picture upload dialogue opens, but the picture isn't added ❌
• Changing Dual stream Presenter thumbnail for a published event afterwards with a newly generated Presentation thumbnail ("use for all tracks") -> newly generated Presentation thumbnail is ignored and old Presenter thumbnail still being used ❌
• Changing Dual stream Presentation thumbnail for a published event afterwards with a newly generated Presenter thumbnail ("use for all tracks") -> newly generated Presenter thumbnail is ignored and old Presentation thumbnail still being used ❌
• Thumbnail picture isn't shown in correct resolution in Paella 8 (probably a Paella player issue and not related to this issue) ❌

To summarize my observations: changing the thumbnails for an already published event and the handling of thumbnails for dual stream events has some issues and needs improvements.