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/app.py

@@ -17,7 +17,6 @@ from bp import (
     register_page_cart,
     register_cart_global,
     register_page_admin,
-    register_fragments,
     register_actions,
     register_data,
     register_inbox,
@@ -141,7 +140,9 @@ def create_app() -> "Quart":
     app.jinja_env.globals["cart_quantity_url"] = lambda product_id: f"/quantity/{product_id}/"
     app.jinja_env.globals["cart_delete_url"] = lambda product_id: f"/delete/{product_id}/"
 
-    app.register_blueprint(register_fragments())
+    from shared.sx.handlers import auto_mount_fragment_handlers
+    auto_mount_fragment_handlers(app, "cart")
+
     app.register_blueprint(register_actions())
     app.register_blueprint(register_data())
     app.register_blueprint(register_inbox())