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>
giles
418ac9424f