add fiat balance

app/policies/fiat_balance_policy.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+class FiatBalancePolicy < ApplicationPolicy
+  class Scope < Scope
+    def resolve
+      return scope.none if user.nil?
+
+      scope.where(user_id: user.id)
+    end
+  end
+end