Fan the unread room notice out from a job

Since the unread rooms stream was scoped per user, posting a message
publishes the room id once to each member of the room. Each publish is a
round trip to Redis, made one after another inside the request, so the time
to post a message grows with the size of the room and in a big room is far
more than the rest of the request.

The fanout now runs in Message::BroadcastUnreadRoomJob, so the poster no
longer waits for it. Who gets the notice and what it says are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NuTtwJKhb77Dv7EQqv2C3C

test/controllers/messages_controller_test.rb

@@ -60,18 +60,32 @@ class MessagesControllerTest < ActionDispatch::IntegrationTest
   test "creating a message broadcasts unread room to each member" do
     @room.users.each do |member|
       assert_broadcasts UnreadRoomsChannel.stream_name_for(member.id), 1 do
-        post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one #{member.id}", client_message_id: member.id } }
+        perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+          post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one #{member.id}", client_message_id: member.id } }
+        end
       end
     end
   end
 
+  test "creating a message leaves the unread fanout to a job" do
+    member = @room.users.excluding(users(:david)).first
+
+    assert_no_broadcasts UnreadRoomsChannel.stream_name_for(member.id) do
+      post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+    end
+
+    assert_enqueued_with job: Message::BroadcastUnreadRoomJob, args: [ Message.last ]
+  end
+
   test "creating a message doesn't broadcast unread room to non-members" do
     outsiders = User.where.not(id: @room.users.map(&:id))
     assert outsiders.any?, "need someone outside the room for this test to mean anything"
 
     outsiders.each do |outsider|
       assert_no_broadcasts UnreadRoomsChannel.stream_name_for(outsider.id) do
-        post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+        perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+          post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+        end
       end
     end
   end