Storage backend and volume normalization groundwork

Storage backend and volume information is currently spread across etcd and the storage-proxy configuration files. Lay the groundwork for normalizing it into the database.

Scope:

- Write the BEP covering the normalized schema and the heartbeat registration rules.
- Add the storage backend, volume and mount tables, and the vfolder storage volume reference.
- Implement mount and backend health checks in the storage proxy, and update the backend status from the resulting health events.

Service registration and heartbeat work is tracked in a separate epic. Migrating the existing etcd data and retiring the legacy configuration are tracked separately as well.

JIRA Issue: BA-7620

Start by locating the storage-proxy configuration files and the existing etcd-backed storage and volume definitions, then review the project's BEP and schema conventions. Done means the BEP covers the normalized schema and heartbeat rules, the backend, volume, mount, and vfolder reference tables exist, and health events update backend status; service registration and migration remain separate work.

Written by the indexing model from the issue text.