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>

cart/sxc/pages/cart.sx

@@ -1,25 +1,43 @@
 ;; Cart app defpage declarations.
+;; All data fetching via (service ...) IO primitives, no Python helpers.
 
 (defpage cart-overview
   :path "/"
   :auth :public
   :layout :root
-  :content (overview-content))
+  :data (service "cart-page" "overview-data")
+  :content (~cart-overview-content
+    :page-groups page-groups
+    :cart-url-base cart-url-base))
 
 (defpage page-cart-view
   :path "/<page_slug>/"
   :auth :public
   :layout :cart-page
-  :content (page-cart-content))
+  :data (service "cart-page" "page-cart-data")
+  :content (~cart-page-cart-content
+    :cart-items cart-items
+    :cal-entries cal-entries
+    :ticket-groups ticket-groups
+    :summary (~cart-summary-from-data
+      :item-count (get summary "item_count")
+      :grand-total (get summary "grand_total")
+      :symbol (get summary "symbol")
+      :is-logged-in (get summary "is_logged_in")
+      :checkout-action (get summary "checkout_action")
+      :login-href (get summary "login_href")
+      :user-email (get summary "user_email"))))
 
 (defpage cart-admin
   :path "/<page_slug>/admin/"
   :auth :admin
   :layout :cart-admin
-  :content (cart-admin-content))
+  :content (~cart-admin-content))
 
 (defpage cart-payments
   :path "/<page_slug>/admin/payments/"
   :auth :admin
   :layout (:cart-admin :selected "payments")
-  :content (cart-payments-content))
+  :data (service "cart-page" "payments-data")
+  :content (~cart-payments-content
+    :page-config page-config))