Horizon Lite: Generate per-ledger indices for accounts

[Shaptic]

We currently have per-checkpoint indices, but per-ledger ones would be useful for comparisons.

We should use the single-process index builder to minimize deployment complexity until #4566 is resolved. You can achieve this via the accounts_by_ledger module on the indexer.

Acceptance Criteria

• Either (a) the existing S3 bucket is split into two folders or (b) a separate S3 bucket is created for these indices.
• 24 hours of indices on a recent, contiguous ledger range are on said S3 bucket/folder.
• A "live" indexer is continuously running on the rest of the above ledger range.