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

The Rust port creates index_messages_on_room_id_and_created_at on boot,
under the same name and on the same columns, with CREATE INDEX IF NOT
EXISTS. On a database the port opened before this migration ran,
db:prepare stopped with "index ... already exists" and the app didn't
start. With if_not_exists the migration skips an index that is already
there and creates it everywhere else.

Databases that already ran the migration don't run it again, and the
dumped schema is the same, so db/schema.rb doesn't change.

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

db/migrate/20261004225000_add_room_id_and_created_at_index_to_messages.rb

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