Check for a second page of messages without counting the room

The original room shows its welcome box until it holds more than a page of
messages, and paged? answered that with a COUNT(*) over every message in
the room on each visit. Asking whether there is a row past the first page
answers the same question while reading at most a page of index entries.

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

app/models/message/pagination.rb

@@ -49,7 +49,7 @@ module Message::Pagination
     end
 
     def paged?
-      count > PAGE_SIZE
+      offset(PAGE_SIZE).exists?
     end
   end
 end