br: support snapshot backup repository layout

## Enhancement

Support a repo-scoped snapshot backup layout in BR so multiple snapshot backups can share one storage prefix safely.

### Problem

Today, BR snapshot backup expects an empty `--storage` path and writes `backupmeta`, `backup.lock`, and SSTs directly into that path. In very large S3-compatible repositories, creating a brand-new prefix and immediately writing a large number of objects can trigger throttling or timeouts.

### Proposed enhancement

- Treat `--storage` as a repository root for snapshot backups.
- Reserve `_meta/` and `_data/` namespaces for metadata and large data.
- Allocate a stable backup ID per snapshot backup attempt.
- Shard snapshot SST data by store under `_data/snapshot///`.
- Add repo-level metadata and pending markers for resume, listing, delete, and cleanup flows.