Enforce the storage volume reference on vfolders

The storage volume reference on virtual folders is added nullable together with the schema, because at that point nothing has populated it yet. Once the migration has run, every folder that should have one does, and the column can be constrained.

Two kinds of folder are exempt:

- Unmanaged folders, which point at a raw filesystem path rather than at a volume.
- Folders in a terminal deletion state, where the data is already gone.

Folders awaiting deletion or being deleted are not exempt: their data still exists and the reference is still needed.

Design: BEP-1078 (Storage Proxy Enhancement).

JIRA Issue: BA-7695

Start with the virtual-folder and storage-volume schema and migration entry points, then read BEP-1078 for the intended lifecycle rules. Check how unmanaged folders and terminal deletion states are represented, and verify that awaiting-deletion and deleting folders retain the required reference. Done means the schema constraint is enforced without rejecting the two documented exemptions.

Written by the indexing model from the issue text.