Create the room and update time index only where it is missing

The Rust port creates index_messages_on_room_id_and_updated_at on boot,
under the same name, with CREATE INDEX IF NOT EXISTS
(basecamp/once-campfire-rust#45). On a database the port has opened,
this migration stopped db:prepare with "index ... already exists" and
the app didn't start. With if_not_exists it skips an index that is
already there and creates it everywhere else, and the dumped schema
stays the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LwoX7uRy5vFZSNKJhSqMSG

db/migrate/20261005022000_add_room_id_and_updated_at_index_to_messages.rb

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