Migrate storage configuration from etcd to the database via a manager CLI command

Move the existing storage configuration into the normalized tables. This runs as a manager CLI subcommand rather than an Alembic migration, so that it can be rehearsed with a dry run and repeated; Alembic only creates and drops schema.

- Pull volume and backend records from the running storage proxies. etcd holds no volume or backend data, so the proxies are the only source; doing it here means an operator does not have to wait for every proxy to be upgraded to the new heartbeat.
- Split the virtual folder host string into a storage volume reference. A host naming a volume that no longer exists still gets a record, so the reference stays valid.
- Convert the default host setting to a storage volume reference.
- Seed the resource group to volume table with every resource group and every volume. Resource groups place no restriction on volumes today, so seeding preserves current behaviour; without it no session could mount anything after the migration.
- Be idempotent and offer a dry run. Rolling back is not covered by an Alembic downgrade, so the command must be safe to re-run and must leave the etcd data intact.

Design: BEP-1078 (Storage Proxy Enhancement).

JIRA Issue: BA-7693

Read BEP-1078 and locate the manager CLI entry point and normalized storage tables. Trace how running storage proxies expose volume and backend records, then define completion around an idempotent, dry-run-capable command that preserves etcd data and seeds all required references.

Written by the indexing model from the issue text.