Add upsert behaviour to attach_child (revive soft-deleted relations)

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

services/relationships.py

@@ -18,7 +18,44 @@ async def attach_child(
 ) -> ContainerRelation:
     """
     Create a ContainerRelation and emit container.child_attached event.
+
+    Upsert behaviour: if a relation already exists (including soft-deleted),
+    revive it instead of inserting a duplicate.
     """
+    # Check for existing (including soft-deleted)
+    existing = await session.scalar(
+        select(ContainerRelation).where(
+            ContainerRelation.parent_type == parent_type,
+            ContainerRelation.parent_id == parent_id,
+            ContainerRelation.child_type == child_type,
+            ContainerRelation.child_id == child_id,
+        )
+    )
+    if existing:
+        if existing.deleted_at is not None:
+            # Revive soft-deleted relation
+            existing.deleted_at = None
+            if sort_order is not None:
+                existing.sort_order = sort_order
+            if label is not None:
+                existing.label = label
+            await session.flush()
+            await emit_event(
+                session,
+                event_type="container.child_attached",
+                aggregate_type="container_relation",
+                aggregate_id=existing.id,
+                payload={
+                    "parent_type": parent_type,
+                    "parent_id": parent_id,
+                    "child_type": child_type,
+                    "child_id": child_id,
+                },
+            )
+            return existing
+        # Already attached and active — no-op
+        return existing
+
     if sort_order is None:
         max_order = await session.scalar(
             select(func.max(ContainerRelation.sort_order)).where(