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>

services/navigation.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from shared.models.menu_node import MenuNode
+
+
+async def get_navigation_tree(session: AsyncSession) -> list[MenuNode]:
+    """
+    Return top-level menu nodes ordered by sort_order.
+
+    All apps call this directly (shared DB) — no more HTTP API.
+    """
+    result = await session.execute(
+        select(MenuNode)
+        .where(MenuNode.deleted_at.is_(None), MenuNode.depth == 0)
+        .order_by(MenuNode.sort_order.asc(), MenuNode.id.asc())
+    )
+    return list(result.scalars().all())
+
+
+async def rebuild_navigation(session: AsyncSession) -> None:
+    """
+    Rebuild menu_nodes from container_relations.
+
+    Called by event handlers when relationships change.
+    Currently a no-op placeholder — menu nodes are managed directly
+    by the admin UI. When the full relationship-driven nav is needed,
+    this will sync ContainerRelation -> MenuNode.
+    """
+    pass