Auto-mount fragment handlers: eliminate fragment blueprint boilerplate across all 8 services

Fragment read API is now fully declarative — every handler is a defhandler
s-expression dispatched through one shared auto_mount_fragment_handlers()
function. Replaces 8 near-identical blueprint files (~35 lines each) with
a single function call per service. Events Python handlers (container-cards,
account-page) extracted to a standalone module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cart/bp/__init__.py

@@ -2,7 +2,6 @@ from .cart.overview_routes import register as register_cart_overview
 from .cart.page_routes import register as register_page_cart
 from .cart.global_routes import register as register_cart_global
 from .page_admin.routes import register as register_page_admin
-from .fragments import register_fragments
 from .actions import register_actions
 from .data import register_data
 from .inbox import register_inbox