Nest a draft container tree under another container

Nests an existing DRAFT tree under an existing container, so you can build a sub-structure separately, review it, then slot it into place.

The sibling of PUT /{structureId}/owner: that gives a draft an owner directly, this gives it a parent and it inherits whatever the parent has. If the target container belongs to a program or specialisation, the whole moved subtree becomes part of that curriculum; if the target is itself a draft, the subtree stays a draft.

Only a draft ROOT can be moved. A container already in a live curriculum is not re-parented here. A target inside the tree being moved is rejected, since that would make a cycle.

Pass sort to set its position among the target’s children.