add testing environment

db/migrate/20240120192055_create_account_linked_accounts.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class CreateAccountLinkedAccounts < ActiveRecord::Migration[7.1]
+  def change
+    create_table :account_linked_accounts do |t|
+      t.references :account, null: false, foreign_key: true
+      t.references :linked_account, null: false, foreign_key: { to_table: :accounts }
+      t.text :account_type, null: false
+
+      t.index %i[account_id linked_account_id], unique: true
+
+      t.timestamps
+    end
+  end
+end