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>

models/container_relation.py

@@ -0,0 +1,38 @@
+from datetime import datetime
+from typing import Optional
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy import Integer, String, DateTime, Index, UniqueConstraint, func
+from shared.db.base import Base
+
+
+class ContainerRelation(Base):
+    __tablename__ = "container_relations"
+
+    __table_args__ = (
+        UniqueConstraint(
+            "parent_type", "parent_id", "child_type", "child_id",
+            name="uq_container_relations_parent_child",
+        ),
+        Index("ix_container_relations_parent", "parent_type", "parent_id"),
+        Index("ix_container_relations_child", "child_type", "child_id"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    parent_type: Mapped[str] = mapped_column(String(32), nullable=False)
+    parent_id: Mapped[int] = mapped_column(Integer, nullable=False)
+    child_type: Mapped[str] = mapped_column(String(32), nullable=False)
+    child_id: Mapped[int] = mapped_column(Integer, nullable=False)
+
+    sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    label: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False,
+    )
+    deleted_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )