media assets: save more metadata for videos.

Include the following metadata tags for videos:

* FFmpeg:MajorBrand (e.g. mp42)
* FFmpeg:PixFmt (e.g. yuv420p. Indicates bit depth and color subsampling mode)
* FFmpeg:FrameCount (e.g. total number of frames in the video)
* FFmpeg:VideoCodec (e.g. h264)
* FFmpeg:VideoProfile (e.g. Main)
* FFmpeg:AudioCodec (e.g. AAC)
* FFmpeg:AudioProfile (e.g. LC)
* FFmpeg:AudioLayout (e.g. stereo)
* FFmpeg:AudioBitRate (e.g. 128kb/s)

app/logical/ffmpeg.rb

@@ -83,6 +83,10 @@ class FFmpeg
     frame_count / duration
   end
 
+  def major_brand
+    metadata.dig(:format, :tags, :major_brand)
+  end
+
   def pix_fmt
     video_stream[:pix_fmt]
   end
@@ -99,6 +103,14 @@ class FFmpeg
     metadata[:streams].to_a.select { |stream| stream[:codec_type] == "video" }
   end
 
+  def audio_codec
+    audio_stream[:codec_name]
+  end
+
+  def audio_stream
+    audio_streams.first || {}
+  end
+
   def audio_streams
     metadata[:streams].to_a.select { |stream| stream[:codec_type] == "audio" }
   end