Keep the room_id index and date the migration in UTC

The composite index doesn't serve everything the room_id one did. In
SQLite that index is (room_id, rowid), so it also reads a room's messages
in id order, and it is the narrower one to count a room with. Keeping both,
as memberships already does, leaves those lookups as they were.

The migration was named after local time, two hours ahead of the UTC
timestamp Rails generates; it now carries a UTC one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NuTtwJKhb77Dv7EQqv2C3C

db/migrate/20261004225000_add_room_id_and_created_at_index_to_messages.rb

@@ -1,6 +1,5 @@
 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: 2026_10_05_010000) do
+ActiveRecord::Schema[8.2].define(version: 2026_10_04_225000) do
   create_table "accounts", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.text "custom_styles"
@@ -102,6 +102,7 @@ ActiveRecord::Schema[8.2].define(version: 2026_10_05_010000) do
     t.datetime "updated_at", null: false
     t.index ["creator_id"], name: "index_messages_on_creator_id"
     t.index ["room_id", "created_at"], name: "index_messages_on_room_id_and_created_at"
+    t.index ["room_id"], name: "index_messages_on_room_id"
   end
 
   create_table "push_subscriptions", force: :cascade do |t|