[Feature] Add support for image deduplication

[sleetx]

Stash already has a duplicate checker tool (Settings > Tools > Scene Duplicate Checker).

Let's extend that to support images as well. Assuming it would require phash generation for images.

Various users have brought this up on Discord, to positive feedback. Consensus seems to be that updating the UI/UX to support potentially numerous images would be a bigger challenge than the backend.