Ops API: list_previous_configurations + restore_previous_configuration

Two new Ops API operations to surface and restore the configuration backup history:

## `list_previous_configurations` (CORE-2883)

Return the list of configuration backups from `~/hdb/backup/`, including their timestamps, so operators can identify which version to restore.

```json
{ "operation": "list_previous_configurations" }
```

Response: array of `{ timestamp, path, description }` (or similar).

## `restore_previous_configuration` (CORE-2884)

Restore a previous configuration version by timestamp. Copies the designated file from `~/hdb/backup/` to `~/hdb/harperdb-config.yaml` (optionally with `replicated: true` for cluster-wide restore).

```json
{
"operation": "restore_previous_configuration",
"timestamp": "2025-09-16T12:34:56Z"
}
```

## Acceptance criteria

- `list_previous_configurations` returns a usable list of available backups with timestamps that can be passed directly to `restore_previous_configuration`.
- `restore_previous_configuration` succeeds and triggers a config reload (restart if necessary).
- Both operations are documented in the Ops API docs.
- Error handling: clear messages for "no backup at that timestamp" / "backup file corrupt".

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Jira fields: **Feature Type:** ESS Request · **Business Impact:** Operational efficiency *(both suggested)*

🤖 Filed by Claude on behalf of Kris.