Watermark AI-generated image files

@nextcloud/integration

We could watermark all the images generated by the task processing providers. If we do it at the task processing level, it saves us the effort of implementing it in all providers.

• Visible watermark: We could insert a little image (logo + text) in a corner of images. Something like the assistant logo and a label like "AI-generated"
• Invisible watermark
  • We can store information as EXIF data. We could store a comment. We might be able to use the Php imagemagick extension or the GD one. It would better than using lsolesen/pel which is abandonned.
  • There might be a way to embed something invisible in the image itself. For example: https://github.com/SebaOfficial/Steganography

This could be done right before storing the images in appdata, when the task results are set by the provider.

For context: the watermarking is a requirement for the AI act, even if it can easily be erased we have to implement it to the best of our abilities. If there is an easy method to make an invisible watermark, we have to do it, and if it is not realistic a simpler method like EXIF / ID3 is likely sufficient.

Start at the task-processing path where provider results are set before images are stored in appdata. Compare the proposed visible watermark with EXIF metadata using the PHP ImageMagick or GD extensions, and determine whether invisible embedding is realistic. Done means generated images receive a watermark that meets the stated AI Act requirement.