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.

app/views/cards/_messages.html.erb

@@ -1,7 +1,11 @@
 <%= messages_tag(card) do %>
   <% if card.published? %>
     <%= render partial: "cards/comments/comment", collection: card.comments.preloaded.chronologically, cached: true %>
-    <%= render "cards/comments/new", card: card %>
+    <% if Fizzy.saas? %>
+      <%= render "cards/comments/saas/new", card: card %>
+    <% else %>
+      <%= render "cards/comments/new", card: card %>
+    <% end %>
 
     <%= render "cards/comments/watchers", card: card %>
   <% end %>

app/views/cards/container/footer/_create.html.erb

@@ -13,5 +13,7 @@
       <span>Create and add another</span>
     <% end %>
   </div>
+
+  <%= render "cards/container/footer/saas/storage_limit_notice" if Fizzy.saas? %>
 </div>
 

app/views/cards/drafts/_container.html.erb

@@ -26,5 +26,9 @@
     </div>
   <% end %>
 
-  <%= render "cards/container/footer/create", card: card %>
+  <% if Fizzy.saas? %>
+    <%= render "cards/container/footer/saas/create", card: card %>
+  <% else %>
+    <%= render "cards/container/footer/create", card: card %>
+  <% end %>
 </section>