Add 1GB storage limit with self-host notice for SaaS

Limit free storage to 1GB on the SaaS version. When exceeded, card
publishing, comment creation, and JSON card creation are blocked,
and the card footer shows a "self-host Fizzy for unlimited storage"
notice instead of the create buttons. A nearing-limit warning
appears when usage exceeds 500MB.

Uses the same SaaS engine patterns as the removed billing system:
model concern on Account, controller concerns included via
config.to_prepare, view partials in saas/ with Fizzy.saas? guards
in the main app.

saas/lib/fizzy/saas/engine.rb

@@ -133,10 +133,14 @@ module Fizzy
       end
 
       config.to_prepare do
+        ::Account.include Account::StorageLimited
         ::Identity.include Authorization::Identity, Identity::Devices
         ::Session.include Session::Devices
         ::Signup.prepend Signup
         ApplicationController.include Authorization::Controller
+        CardsController.include(Card::StorageLimited::Creation)
+        Cards::CommentsController.include(Card::StorageLimited::Commenting)
+        Cards::PublishesController.include(Card::StorageLimited::Publishing)
 
         Notification.register_push_target(:native)