add fiat balance

db/migrate/20210812011039_create_fiat_balances.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+class CreateFiatBalances < ActiveRecord::Migration[6.1]
+  def change
+    create_table(:fiat_balances) do |t|
+      t.references(:user, null: false, foreign_key: true)
+      t.monetize(:amount)
+
+      t.timestamps
+    end
+  end
+end