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

test/controllers/messages_caching_test.rb

@@ -27,6 +27,22 @@ class MessagesCachingTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "a page of messages loads whether their previews were made along with the messages, not one at a time" do
+    room = rooms(:watercooler)
+    2.times do |copy|
+      { "moon.jpg" => "image/jpeg", "alpha-centuri.mov" => "video/quicktime" }.each do |file, content_type|
+        room.messages.create_with_attachment! creator: users(:david), client_message_id: "#{copy}-#{file}", attachment: fixture_file_upload(file, content_type)
+      end
+    end
+
+    assert_no_queries_match(/"active_storage_\w+"\."(id|blob_id|record_id)" = \?/) do
+      get room_messages_url(room)
+    end
+    assert_response :success
+    assert_select "img[src*='moon.jpg']", count: 2
+    assert_select "video[poster]", count: 2
+  end
+
   private
     def with_memory_cache
       old_cache = Rails.cache