Backfill without a cloud account, 30× faster

Historical backfill now runs against a cloud object store hosted locally, so the standard export tooling behaves exactly as it would against a remote bucket: no account, no external dependency.

• The cloud’s own storage software runs on a local machine behind the same S3 interface the export tools expect.
• Six parallel exporters wrote the full testnet history at more than thirty times the previous speed.
• Ledgers are exported to files once and reused, replacing repeated CPU-bound replay.