Index messages by room and creation time

Every page of a room's messages (opening the room, scrolling back, the
refresh after reconnecting, the bot API) selects WHERE room_id = ? ORDER BY
created_at LIMIT 40. With only the room_id index, SQLite reads every
message in the room and sorts them in a temporary B-tree to return 40, so
these requests grow with the room's history.

A composite index on (room_id, created_at) lets SQLite read the 40 rows in
order straight from the index. It covers every lookup the room_id index
served, so that one goes.

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

db/migrate/20261005010000_add_room_id_and_created_at_index_to_messages.rb

@@ -0,0 +1,6 @@
+class AddRoomIdAndCreatedAtIndexToMessages < ActiveRecord::Migration[8.2]
+  def change
+    add_index :messages, %i[ room_id created_at ]
+    remove_index :messages, :room_id
+  end
+end