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

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_022000) do
   create_table "accounts", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.text "custom_styles"
@@ -101,6 +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", "updated_at"], name: "index_messages_on_room_id_and_updated_at"
     t.index ["room_id"], name: "index_messages_on_room_id"
   end