Find the messages a refresh replaces through an index

Every room visit and every reconnection asks for the messages updated
since the page was rendered. That query filtered the room by updated_at
and sorted it by created_at, so SQLite walked every message in the room:
about 220-360 ms in a room of a million messages. An index on
(room_id, updated_at) finds the few updated messages directly.

Sorting on +created_at keeps SQLite on that index once messages are also
indexed by (room_id, created_at): with both, the planner otherwise walks
the room in creation order looking for updated rows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JVFo3Lt9T8M5NR7KxVvsZ2

db/migrate/20261005022000_add_room_id_and_updated_at_index_to_messages.rb

@@ -0,0 +1,5 @@
+class AddRoomIdAndUpdatedAtIndexToMessages < ActiveRecord::Migration[8.2]
+  def change
+    add_index :messages, %i[ room_id updated_at ]
+  end
+end