Write the BEP for storage proxy enhancement

Write BEP-1078, the design for normalizing storage proxy, backend and volume information into the database. It builds on BEP-1046 rather than restating the service catalog.

The document defines the entity model across storage proxies, agents, storage backends, storage volumes, resource groups, virtual folders and sessions; where state is managed; how records are created and change from heartbeats; and which component calls which.

JIRA Issue: BA-7652

Start by reading BEP-1046 so BEP-1078 builds on it without restating the service catalog. Define the entity model for storage proxies, agents, storage backends, storage volumes, resource groups, virtual folders, and sessions, including state ownership, heartbeat-driven record changes, and component calls. Done means the design is captured in BEP-1078 and covers each requested area.

Written by the indexing model from the issue text.