Domain isolation: replace cross-domain imports with service calls

Replace direct Post, Calendar, CalendarEntry model queries and glue
lifecycle imports with typed service calls. Cart registers all 4
services via domain_services_fn with has() guards.

Key changes:
- app.py: use domain_services_fn, Post query → services.blog
- api.py: Calendar/CalendarEntry → services.calendar
- checkout: glue order_lifecycle → services.calendar.claim/confirm
- calendar_cart: CalendarEntry → services.calendar.pending_entries()
- page_cart: Post/Calendar queries → services.blog/calendar
- global_routes: glue imports → service calls

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

services/__init__.py

@@ -0,0 +1,25 @@
+"""Cart app service registration."""
+from __future__ import annotations
+
+
+def register_domain_services() -> None:
+    """Register services for the cart app.
+
+    Cart owns: Order, OrderItem.
+    Standard deployment registers all 4 services as real DB impls
+    (shared DB). For composable deployments, swap non-owned services
+    with stubs from shared.services.stubs.
+    """
+    from shared.services.registry import services
+    from shared.services.blog_impl import SqlBlogService
+    from shared.services.calendar_impl import SqlCalendarService
+    from shared.services.market_impl import SqlMarketService
+    from shared.services.cart_impl import SqlCartService
+
+    services.cart = SqlCartService()
+    if not services.has("blog"):
+        services.blog = SqlBlogService()
+    if not services.has("calendar"):
+        services.calendar = SqlCalendarService()
+    if not services.has("market"):
+        services.market = SqlMarketService()