Track post creator as uploader for thumbnail generation

[Nothing4You]

Requirements

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Is your proposal related to a problem?

Automatically generated thumbnails are currently not associated with the user triggering their creation.
This prevents safely deleting thumbnails linked to a user's posts.

Describe the solution you'd like.

Similar to how local user media uploads are already tracked it would be good to store thumbnail URLs in a similar way. Unlike regular user uploads, thumbnail creation can also be triggered by remote users, so this will require matching person.id rather than local_user.id.

Describe alternatives you've considered.

Expensive SQL queries to determine whether other references to the same image exist.

Additional context

Thumbnails are already tracked in the local_image table, but there is no user associated with them.

[Nothing4You]

Optionally, this could also track post ids for thumbnail generation, which allows for removal of locally generated nsfw thumbnails for posts that have been removed for being marked nsfw on nsfw-disabled instances.