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/services/__init__.py

@@ -12,3 +12,6 @@ def register_domain_services() -> None:
     from shared.services.cart_impl import SqlCartService
 
     services.cart = SqlCartService()
+
+    from .cart_page import CartPageService
+    services.register("cart_page", CartPageService())