Decoupling audit: remove cart API, update shared submodule

- Delete bp/cart/api.py (dead internal API endpoint)
- Remove registration from bp/__init__.py and app.py
- Update shared submodule

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

bp/__init__.py

@@ -1,6 +1,5 @@
 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 .cart.api import register as register_cart_api
 from .order.routes import register as register_order
 from .orders.routes import register as register_orders