Domain isolation: typed contracts, service registry, and composable wiring

Add typed service contracts (Protocols + frozen DTOs) in shared/contracts/
for cross-domain communication. Each domain exposes a service interface
(BlogService, CalendarService, MarketService, CartService) backed by SQL
implementations in shared/services/. A singleton registry with has() guards
enables composable startup — apps register their own domain service and
stubs for absent domains.

Absorbs glue layer: navigation, relationships, event handlers (login,
container, order) now live in shared/ with has()-guarded service calls.
Factory gains domain_services_fn parameter for per-app service registration.

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

events/handlers/container_handlers.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from shared.events import register_handler
+from shared.models.domain_event import DomainEvent
+from shared.services.navigation import rebuild_navigation
+
+
+async def on_child_attached(event: DomainEvent, session: AsyncSession) -> None:
+    await rebuild_navigation(session)
+
+
+async def on_child_detached(event: DomainEvent, session: AsyncSession) -> None:
+    await rebuild_navigation(session)
+
+
+register_handler("container.child_attached", on_child_attached)
+register_handler("container.child_detached", on_child_detached)