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(""),
+        }

account/sxc/pages/__init__.py

@@ -1,13 +1,12 @@
-"""Account defpage setup — registers layouts, page helpers, and loads .sx pages."""
+"""Account defpage setup — registers layouts and loads .sx pages."""
 from __future__ import annotations
 
 from typing import Any
 
 
 def setup_account_pages() -> None:
-    """Register account-specific layouts, page helpers, and load page definitions."""
+    """Register account-specific layouts and load page definitions."""
     _register_account_layouts()
-    _register_account_helpers()
     _load_account_page_files()
 
 
@@ -90,50 +89,3 @@ def _as_sx_nav(ctx: dict) -> Any:
     return _as_sx(ctx.get("account_nav"))
 
 
-# ---------------------------------------------------------------------------
-# Page helpers
-# ---------------------------------------------------------------------------
-
-def _register_account_helpers() -> None:
-    from shared.sx.pages import register_page_helpers
-
-    register_page_helpers("account", {
-        "newsletters-data": _h_newsletters_data,
-    })
-
-
-async def _h_newsletters_data(**kw):
-    """Fetch newsletter data — returns dict merged into defpage env."""
-    from quart import g
-    from sqlalchemy import select
-    from shared.models import UserNewsletter
-    from shared.models.ghost_membership_entities import GhostNewsletter
-
-    result = await g.s.execute(
-        select(GhostNewsletter).order_by(GhostNewsletter.name)
-    )
-    all_newsletters = result.scalars().all()
-
-    sub_result = await g.s.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,
-        })
-
-    account_url = getattr(g, "_account_url", None)
-    if account_url is None:
-        from shared.infrastructure.urls import account_url as _account_url
-        account_url = _account_url
-    account_url_str = account_url("") if callable(account_url) else str(account_url or "")
-
-    return {"newsletter-list": newsletter_list, "account-url": account_url_str}

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))