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

lib/rails_ext/time_limited_video_previewer.rb

@@ -0,0 +1,27 @@
+# Rails waits for ffmpeg however long it takes, and a video's preview is drawn inside the request that posts it.
+# This previewer gives ffmpeg a wall-clock limit, kills it past that, and reports a failed preview, so the message
+# is posted without one.
+class TimeLimitedVideoPreviewer < ActiveStorage::Previewer::VideoPreviewer
+  TIME_LIMIT = 10 # seconds
+
+  private
+    def capture(*argv, to:)
+      to.binmode
+
+      open_tempfile do |err|
+        IO.popen(argv, in: IO::NULL, err: err) do |out|
+          Timeout.timeout(TIME_LIMIT) { IO.copy_stream(out, to) }
+        rescue Timeout::Error
+          Process.kill :KILL, out.pid
+          raise ActiveStorage::PreviewError, "#{argv.first} took longer than #{TIME_LIMIT} seconds"
+        end
+        err.rewind
+
+        unless $?.success?
+          raise ActiveStorage::PreviewError, "#{argv.first} failed (status #{$?.exitstatus}): #{err.read.to_s.chomp}"
+        end
+      end
+
+      to.rewind
+    end
+end