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/order_handlers.py

@@ -0,0 +1,22 @@
+from __future__ import annotations
+
+import logging
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from shared.events import register_handler
+from shared.models.domain_event import DomainEvent
+
+log = logging.getLogger(__name__)
+
+
+async def on_order_created(event: DomainEvent, session: AsyncSession) -> None:
+    log.info("order.created: order_id=%s", event.payload.get("order_id"))
+
+
+async def on_order_paid(event: DomainEvent, session: AsyncSession) -> None:
+    log.info("order.paid: order_id=%s", event.payload.get("order_id"))
+
+
+register_handler("order.created", on_order_created)
+register_handler("order.paid", on_order_paid)