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/__init__.py

@@ -0,0 +1,8 @@
+"""Shared event handlers (replaces glue.setup.register_glue_handlers)."""
+
+
+def register_shared_handlers():
+    """Import handler modules to trigger registration. Call at app startup."""
+    import shared.events.handlers.container_handlers  # noqa: F401
+    import shared.events.handlers.login_handlers  # noqa: F401
+    import shared.events.handlers.order_handlers  # noqa: F401

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)

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)

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)