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

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.2].define(version: 2025_12_12_154340) do
+ActiveRecord::Schema[8.2].define(version: 2026_10_05_010000) do
   create_table "accounts", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.text "custom_styles"
@@ -101,7 +101,7 @@ ActiveRecord::Schema[8.2].define(version: 2025_12_12_154340) do
     t.integer "room_id", null: false
     t.datetime "updated_at", null: false
     t.index ["creator_id"], name: "index_messages_on_creator_id"
-    t.index ["room_id"], name: "index_messages_on_room_id"
+    t.index ["room_id", "created_at"], name: "index_messages_on_room_id_and_created_at"
   end
 
   create_table "push_subscriptions", force: :cascade do |t|