feat(indexer): add historical event backfill catch-up mode

[0xBernny]

closes #256

Summary

Related Issue

Closes #256

Complexity

• Trivial (typo, docs, small refactor)
• [ ✔️] Medium (new feature, bug fix)
• High (major refactor, breaking change, security-critical)

Changes Made

• Created a new ingestion utility class inside indexer/src/ingestion/catchup.ts to check ledger gaps on startup
• Implemented historical event fetching logic in batches of 200 ledgers using Horizon's paginated transactions endpoint.
• Added descriptive log outputs for tracking backfill status and remaining progress estimation.

Test Coverage

TypeScript / Jest framework test structure verified

Mocked transaction arrays successfully validate non-streaming pagination limits

# Example:
# cargo test
# running 15 tests
# test result: ok. 15 passed; 0 failed

Details

Test output

Security Considerations

N/A

Checklist

• [ ✔️] Code follows project conventions (see CONTRIBUTING.md)
• [✔️ ] Tests added/updated and passing
• [✔️ ] Documentation updated (if needed)
• [✔️ ] Commit message follows Conventional Commits
• [ ✔️] No secrets or sensitive data in code
• Breaking changes documented (if applicable)

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