feat: per-page carts with overview, page-scoped checkout, and split blueprints (Phase 4)

Splits the monolithic cart blueprint into three: cart_overview (GET /),
page_cart (/<page_slug>/), and cart_global (webhook, return, add).

- New page_cart.py service: get_cart_for_page(), get_calendar_entries_for_page(), get_cart_grouped_by_page()
- clear_cart_for_order() and create_order_from_cart() accept page_post_id for scoping
- Cart app hydrates page_slug via url_value_preprocessor/url_defaults/hydrate_page
- Context processor provides page-scoped cart data when g.page_post exists
- Internal API /internal/cart/summary accepts ?page_slug= for page-scoped counts
- Overview template shows page cards with item counts and totals
- Page cart template reuses show_cart() macro with page-specific header

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

bp/order/routes.py

@@ -72,12 +72,12 @@ def register() -> Blueprint:
             return redirect(order.sumup_hosted_url)
 
         # Otherwise, create a fresh checkout for this order
-        redirect_url = url_for("cart.checkout_return", order_id=order.id, _external=True)
+        redirect_url = url_for("cart_global.checkout_return", order_id=order.id, _external=True)
 
         sumup_cfg = config().get("sumup", {}) or {}
         webhook_secret = sumup_cfg.get("webhook_secret")
 
-        webhook_url = url_for("cart.checkout_webhook", order_id=order.id, _external=True)
+        webhook_url = url_for("cart_global.checkout_webhook", order_id=order.id, _external=True)
         if webhook_secret:
             from urllib.parse import urlencode