Add extend_existing=True to MenuNode and ContainerRelation models

Prevents SQLAlchemy 'table already defined' error if the table gets
registered by a stale glue submodule or cached Docker layer before
the shared model is loaded.

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

models/container_relation.py

@@ -15,6 +15,7 @@ class ContainerRelation(Base):
         ),
         Index("ix_container_relations_parent", "parent_type", "parent_id"),
         Index("ix_container_relations_child", "child_type", "child_id"),
+        {"extend_existing": True},
     )
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)