Implement 'backfill' service

[Otaiki1]

Context

If Horizon missed an event, we need a way to fill specific gaps.

Goal

Add a service to fetch specific ledger ranges and re-index.

Contributor guide

1. Directory: indexer/
2. Create backfill(startLedger, endLedger) method.
3. Ensure it does not collide with active poller (locks).
4. Log progress and handle missing ledgers in Horizon archive.
5. Add CLI wrapper.

References

• Indexer backfill guide.

[chiemezie1]

@chiemezie1 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hey! I'd like to pick up issue #213 — the backfill service.

I'll implement backfill(startLedger, endLedger) in indexer/ with batched Horizon fetches, idempotent re-indexing, a distributed lock to prevent collisions with the active poller, progress logging, graceful skip-and-report for missing ledgers in the archive, and a CLI wrapper with --start, --end, --dry-run, and --force flags.

Can you assign this to me?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @chiemezie1 to this issue.