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/services/__init__.py

@@ -3,9 +3,7 @@ from __future__ import annotations
 
 
 def register_domain_services() -> None:
-    """Register services for the account app.
-
-    Account is a consumer-only dashboard app. It has no own domain.
-    All cross-app data comes via fragments and HTTP data endpoints.
-    """
-    pass
+    """Register services for the account app."""
+    from shared.services.registry import services
+    from .account_page import AccountPageService
+    services.register("account_page", AccountPageService())