Bound the work of previewing an attachment

A video's preview and a picture's thumbnail are made inside the request that posts the message, and
nothing bounded how long either could take.

- The video preview filter also selects any frame from 5 seconds on. Rails' filter takes the second
  frame it selects, which a video with a single keyframe and no scene change only gives at its end, so
  ffmpeg decoded all of it.
- TimeLimitedVideoPreviewer gives ffmpeg 10 seconds of wall-clock time, kills it past that, and reports
  a failed preview, so the message is posted without one.
- Pictures and videos above 250 megapixels, or whose size couldn't be read, get no preview: decoding
  costs in proportion to the pixels, however small the file.
- The view shows a preview only if it was made when the message was posted. Its URL used to make it on
  view, so a preview that failed or was skipped would be attempted again on every view. The cached
  presentation's version goes up, so cached messages pick this up.
- A video's poster is made, when the message is posted, at the size the view shows it. The full-size WebP
  made until now wasn't shown anywhere, and encoding it costs in proportion to the frame's pixels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bj8KnxpTf9sj2Ysa8aLAVa

config/initializers/active_storage.rb

@@ -1,5 +1,20 @@
+require "rails_ext/time_limited_video_previewer"
+
 ActiveSupport.on_load(:active_storage_blob) do
   ActiveStorage::DiskController.after_action only: :show do
     response.set_header("Cache-Control", "max-age=3600, public")
   end
 end
+
+Rails.application.configure do
+  # Rails' filter takes the second of the frames it selects (the first one, keyframes, scene changes), so a video
+  # with a single keyframe and no scene change is decoded to its end. Selecting any frame from 5 seconds on stops
+  # it there.
+  config.active_storage.video_preview_arguments =
+    "-vf 'select=eq(n\\,0)+eq(key\\,1)+gt(scene\\,0.015)+gte(t\\,5),loop=loop=-1:size=2,trim=start_frame=1'" \
+    " -frames:v 1 -f image2"
+
+  config.active_storage.previewers = config.active_storage.previewers.map do |previewer|
+    previewer == ActiveStorage::Previewer::VideoPreviewer ? TimeLimitedVideoPreviewer : previewer
+  end
+end