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>

federation/sxc/pages/social.sx

@@ -1,49 +1,82 @@
 ;; Federation social pages
+;; All data fetching via (service ...) IO primitives, no Python helpers.
 
 (defpage home-timeline
   :path "/social/"
   :auth :login
   :layout :social
-  :content (home-timeline-content))
+  :data (service "federation-page" "home-timeline-data")
+  :content (~federation-timeline-content
+    :items items
+    :timeline-type timeline-type
+    :actor actor))
 
 (defpage public-timeline
   :path "/social/public"
   :auth :public
   :layout :social
-  :content (public-timeline-content))
+  :data (service "federation-page" "public-timeline-data")
+  :content (~federation-timeline-content
+    :items items
+    :timeline-type timeline-type
+    :actor actor))
 
 (defpage compose-form
   :path "/social/compose"
   :auth :login
   :layout :social
-  :content (compose-content))
+  :data (service "federation-page" "compose-data")
+  :content (~federation-compose-content
+    :reply-to reply-to))
 
 (defpage search
   :path "/social/search"
   :auth :public
   :layout :social
-  :content (search-content))
+  :data (service "federation-page" "search-data")
+  :content (~federation-search-content
+    :query query
+    :actors actors
+    :total total
+    :followed-urls followed-urls
+    :actor actor))
 
 (defpage following-list
   :path "/social/following"
   :auth :login
   :layout :social
-  :content (following-content))
+  :data (service "federation-page" "following-data")
+  :content (~federation-following-content
+    :actors actors
+    :total total
+    :actor actor))
 
 (defpage followers-list
   :path "/social/followers"
   :auth :login
   :layout :social
-  :content (followers-content))
+  :data (service "federation-page" "followers-data")
+  :content (~federation-followers-content
+    :actors actors
+    :total total
+    :followed-urls followed-urls
+    :actor actor))
 
 (defpage actor-timeline
   :path "/social/actor/<int:id>"
   :auth :public
   :layout :social
-  :content (actor-timeline-content id))
+  :data (service "federation-page" "actor-timeline-data" :id id)
+  :content (~federation-actor-timeline-content
+    :remote-actor remote-actor
+    :items items
+    :is-following is-following
+    :actor actor))
 
 (defpage notifications
   :path "/social/notifications"
   :auth :login
   :layout :social
-  :content (notifications-content))
+  :data (service "federation-page" "notifications-data")
+  :content (~federation-notifications-content
+    :notifications notifications))