Domain isolation: replace cross-domain imports with service calls

Replace direct Post, MarketPlace, Calendar model queries and HTTP API
calls with typed service calls. Events registers all 4 services via
domain_services_fn with has() guards.

Key changes:
- app.py: use domain_services_fn, Post/Calendar/MarketPlace queries
  → services.blog/calendar/market, HTTP cart API → services.cart
- calendars/markets services: Post → services.blog
- post_associations: Post → services.blog, direct queries → services
- markets routes: remove unused MarketPlace import
- glue imports → shared imports throughout

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

services/__init__.py

@@ -0,0 +1,26 @@
+"""Events app service registration."""
+from __future__ import annotations
+
+
+def register_domain_services() -> None:
+    """Register services for the events app.
+
+    Events owns: Calendar, CalendarEntry, CalendarSlot, TicketType,
+    Ticket, CalendarEntryPost.
+    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.calendar = SqlCalendarService()
+    if not services.has("blog"):
+        services.blog = SqlBlogService()
+    if not services.has("market"):
+        services.market = SqlMarketService()
+    if not services.has("cart"):
+        services.cart = SqlCartService()