Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/models/container_relation.py

@@ -1,7 +1,7 @@
 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 sqlalchemy import Integer, String, DateTime, Index, JSON, UniqueConstraint, func
 from shared.db.base import Base
 
 
@@ -15,6 +15,10 @@ class ContainerRelation(Base):
         ),
         Index("ix_container_relations_parent", "parent_type", "parent_id"),
         Index("ix_container_relations_child", "child_type", "child_id"),
+        Index(
+            "ix_container_relations_relation_type",
+            "relation_type", "parent_type", "parent_id",
+        ),
     )
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
@@ -24,6 +28,9 @@ class ContainerRelation(Base):
     child_type: Mapped[str] = mapped_column(String(32), nullable=False)
     child_id: Mapped[int] = mapped_column(Integer, nullable=False)
 
+    relation_type: Mapped[Optional[str]] = mapped_column(String(64), nullable=True, index=False)
+    metadata_: Mapped[Optional[dict]] = mapped_column("metadata", JSON, nullable=True)
+
     sort_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
     label: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)