fix: enforce email limits for agent invitations

Atomically reserve Chatwoot Cloud account email capacity before sending new agent invitations, including bulk creation, and roll back with HTTP 429 when the limit is exhausted.

lib/custom_exceptions/account.rb

@@ -42,4 +42,18 @@ module CustomExceptions::Account
       I18n.t 'errors.plan_upgrade_required.failed'
     end
   end
+
+  class EmailLimitExceeded < CustomExceptions::Base
+    def message
+      I18n.t('errors.account.email_limit_exceeded')
+    end
+
+    def to_hash
+      { error: message }
+    end
+
+    def http_status
+      :too_many_requests
+    end
+  end
 end