Logging to Sentry

## Context
Catch ingestion errors in real-time.

## Goal
Integrate Sentry into indexer service.

## Contributor guide
1. **Directory:** `indexer/`
2. Initialize Sentry in main.ts.
3. Add custom Sentry tags: ledger, contract_version, event_type.
4. Send errors from Processors with context (event payload).
5. Alert on spike in ingestion errors.

## References
- Sentry Node.js SDK docs.