posts: automatically tag AI-generated on NovelAI posts.

Automatically add the AI-generated tag to posts that have the
`PNG:Software=NovelAI` EXIF attribute.

This is not foolproof because this metadata may get removed if an
AI-generated post is resaved or uploaded to a site that strips EXIF
metadata. It also only works for NovelAI. Currently it detects 29 out of
177 AI-generated uploads on Danbooru.

app/logical/exif_tool.rb

@@ -80,6 +80,11 @@ class ExifTool
       loop_count.in?(0..10)
     end
 
+    # https://danbooru.donmai.us/media_assets?search[metadata][PNG:Software]=NovelAI
+    def is_ai_generated?
+      metadata["PNG:Software"] == "NovelAI"
+    end
+
     # @see http://www.vurdalakov.net/misc/gif/netscape-looping-application-extension
     # @see https://wiki.mozilla.org/APNG_Specification#.60acTL.60:_The_Animation_Control_Chunk
     # @see https://danbooru.donmai.us/posts?tags=-exif:GIF:AnimationIterations=Infinite+animated_gif

app/models/media_asset.rb

@@ -24,7 +24,7 @@ class MediaAsset < ApplicationRecord
   has_many :ai_tags
 
   delegate :metadata, to: :media_metadata
-  delegate :is_non_repeating_animation?, :is_greyscale?, :is_rotated?, to: :metadata
+  delegate :is_non_repeating_animation?, :is_greyscale?, :is_rotated?, :is_ai_generated?, to: :metadata
 
   scope :public_only, -> { where(is_public: true) }
   scope :private_only, -> { where(is_public: false) }

app/models/post.rb

@@ -465,6 +465,7 @@ class Post < ApplicationRecord
       tags << "greyscale" if media_asset.is_greyscale?
       tags << "exif_rotation" if media_asset.is_rotated?
       tags << "non-repeating_animation" if media_asset.is_non_repeating_animation?
+      tags << "ai-generated" if media_asset.is_ai_generated?
 
       tags
     end