ugoira: store frame delays in MediaMetadata model.

Store Ugoira frame delays in the MediaMetadata model as a fake EXIF
field instead of in the PixivUgoiraFrameData model. This way we can get
rid of the PixivUgoiraFrameData model completely. This is a step towards
fixing #5264.

script/fixes/120_migrate_ugoira_frame_data.rb

@@ -0,0 +1,21 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  MediaMetadata.joins(:media_asset).where(media_asset: { file_ext: "zip" }).find_each do |meta|
+    frame_data = PixivUgoiraFrameData.find_by(md5: meta.media_asset.md5)
+
+    if frame_data.nil?
+      puts "Missing frame data: #{meta.media_asset.md5}"
+      next
+    elsif meta.metadata["Ugoira:FrameDelays"] == frame_data.frame_delays
+      next
+    end
+
+    json = meta.metadata.as_json.merge("Ugoira:FrameDelays" => frame_data.frame_delays)
+    meta.update!(metadata: json) if ENV.fetch("FIX", "false").truthy?
+
+    puts meta.as_json
+  end
+end