uploads: fix temp files not being cleaned up quickly enough.

Fix temp files generated during the upload process not being cleaned up quickly enough. This included
downloaded files, generated preview images, and Ugoira video conversions.

Before we relied on `Tempfile` cleaning up files automatically. But this only happened when the
Tempfile object was garbage collected, which could take a long time. In the meantime we could have
hundreds of megabytes of temp files hanging around.

The fix is to explicitly close temp files when we're done with them. But the standard `Tempfile`
class doesn't immediately delete the file when it's closed. So we also have to introduce a
Danbooru::Tempfile wrapper that deletes the tempfile as soon as it's closed.

app/logical/media_file/ugoira.rb

@@ -17,8 +17,9 @@ class MediaFile::Ugoira < MediaFile
   end
 
   def close
-    file.close
-    preview_frame.close
+    super
+    @preview_frame&.close
+    # XXX should clean up `convert` too
   end
 
   def metadata
@@ -52,7 +53,7 @@ class MediaFile::Ugoira < MediaFile
     raise RuntimeError, "can't convert ugoira to webm: no ugoira frame data was provided" unless frame_delays.present?
 
     Danbooru::Archive.extract!(file) do |tmpdir, filenames|
-      output_file = Tempfile.new(["ugoira-conversion", ".webm"], binmode: true)
+      output_file = Danbooru::Tempfile.new(["danbooru-ugoira-conversion-#{md5}-", ".webm"], binmode: true)
 
       # Duplicate last frame to avoid it being displayed only for a very short amount of time.
       last_file_name = File.basename(filenames.last)
@@ -90,8 +91,8 @@ class MediaFile::Ugoira < MediaFile
   private
 
   def preview_frame
-    FFmpeg.new(convert).smart_video_preview
+    @preview_frame ||= FFmpeg.new(convert).smart_video_preview
   end
 
-  memoize :preview_frame, :dimensions, :convert, :metadata
+  memoize :dimensions, :convert, :metadata
 end