feat(backend): fix nsfw checker catch-22

[psychedelicious]

Summary

• Fixes the catch-22 described in #6252.
• Uses better image blurring logic from #6260, the caution image is now centered on the blurred image, and the logic to do this is integrated in the SafetyChecker class.

Related Issues / Discussions

Closes #6252 Supersedes #6260

QA Instructions

• Delete the NSFW checker model from core/convert/stable-diffusion-safety-checker
• Start Invoke
• The toggle in Settings should not be greyed out
• Enable the NSFW checker in settings
• Generate something spicy
• The model should download and then blur the image, adding the caution symbol centered in the output

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)