fix(api): model cover image lost

[psychedelicious]

Summary

Fixes two bugs with model cover images:

• fix(api): add cover image to update model response

  Fixes a bug where the image appears to be reset when editing a model.

  See: https://old.reddit.com/r/StableDiffusion/comments/1cnx40d/invoke_42_control_layers_regional_guidance_w_text/l3asdej/

• fix(api): retain cover image when converting model to diffusers

  We need to retrieve and re-save the image, because a conversion to diffusers creates a new model record, with a new key.

  See: https://old.reddit.com/r/StableDiffusion/comments/1cnx40d/invoke_42_control_layers_regional_guidance_w_text/l3bv152/

Related Issues / Discussions

^ reddit links

QA Instructions

Losing cover image when editing a model:

• Set a model's image (e.g. a LoRA)
• Edit the model in any way (e.g. add a trigger phrase to a LoRA)
• The image should not be lost

Losing cover image when converting a diffusers model:

• Set a model image on a diffusers model
• Convert to diffusers
• The image should not be lost

Merge Plan

n/a

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [ ] Tests added / updated (if applicable)
• [x] Documentation added / updated (if applicable)