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/cart/services/get_cart.py

@@ -18,7 +18,8 @@ async def get_cart(session):
       .where(*filters)
       .order_by(CartItem.created_at.desc())
       .options(
-          selectinload(CartItem.product),  # <-- important bit
+          selectinload(CartItem.product),
+          selectinload(CartItem.market_place),
       )
   )
   return result.scalars().all()