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/login_handlers.py

@@ -0,0 +1,24 @@
+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.registry import services
+
+
+async def on_user_logged_in(event: DomainEvent, session: AsyncSession) -> None:
+    payload = event.payload
+    user_id = payload["user_id"]
+    session_id = payload["session_id"]
+
+    # Adopt cart items (if cart service is registered)
+    if services.has("cart"):
+        await services.cart.adopt_cart_for_user(session, user_id, session_id)
+
+    # Adopt calendar entries (if calendar service is registered)
+    if services.has("calendar"):
+        await services.calendar.adopt_entries_for_user(session, user_id, session_id)
+
+
+register_handler("user.logged_in", on_user_logged_in)