Eliminate Python page helpers from account, federation, and cart

All three services now fetch page data via (service ...) IO primitives
in .sx defpages instead of Python middleman functions.

- Account: newsletters-data → AccountPageService.newsletters_data
- Federation: 8 page helpers → FederationPageService methods
  (timeline, compose, search, following, followers, notifications)
- Cart: 4 page helpers → CartPageService methods
  (overview, page-cart, admin, payments)
- Serializers moved to service modules, thin delegates kept for routes
- ~520 lines of Python page helpers removed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

account/sxc/pages/account.sx

@@ -18,7 +18,7 @@
   :path "/newsletters/"
   :auth :login
   :layout :account
-  :data (newsletters-data)
+  :data (service "account-page" "newsletters-data")
   :content (~account-newsletters-content
     :newsletter-list newsletter-list
     :account-url account-url))