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())

account/services/account_page.py

@@ -0,0 +1,40 @@
+"""Account page data service — provides serialized dicts for .sx defpages."""
+from __future__ import annotations
+
+
+class AccountPageService:
+    """Service for account page data, callable via (service "account-page" ...)."""
+
+    async def newsletters_data(self, session, **kw):
+        """Return newsletter list with user subscription status."""
+        from quart import g
+        from sqlalchemy import select
+        from shared.models import UserNewsletter
+        from shared.models.ghost_membership_entities import GhostNewsletter
+
+        result = await session.execute(
+            select(GhostNewsletter).order_by(GhostNewsletter.name)
+        )
+        all_newsletters = result.scalars().all()
+
+        sub_result = await session.execute(
+            select(UserNewsletter).where(
+                UserNewsletter.user_id == g.user.id,
+            )
+        )
+        user_subs = {un.newsletter_id: un for un in sub_result.scalars().all()}
+
+        newsletter_list = []
+        for nl in all_newsletters:
+            un = user_subs.get(nl.id)
+            newsletter_list.append({
+                "newsletter": {"id": nl.id, "name": nl.name, "description": nl.description},
+                "un": {"newsletter_id": un.newsletter_id, "subscribed": un.subscribed} if un else None,
+                "subscribed": un.subscribed if un else False,
+            })
+
+        from shared.infrastructure.urls import account_url
+        return {
+            "newsletter_list": newsletter_list,
+            "account_url": account_url(""),
+        }