feat(stellar): add Stellar event indexing service (lib/stellar/indexer.ts)

[devfoma]

This pr closes #13

Summary

Implements the Stellar event indexing service to synchronize Stellar ledger events into MongoDB idempotently for monitoring/dashboards.

Changes

• lib/stellar/indexer.ts — Core indexer factory. Supports mock and live Horizon modes, maps events to ChainMove records, handles checkpoints, and stores records in MongoDB safely.
• models/StellarIndexedEvent.ts — Mongoose model for storing indexed events where _id acts as the idempotency key (Stellar operation ID).
• models/StellarIndexerCursor.ts — Mongoose model tracking the paging token / cursor checkpoint for the stream.
• lib/stellar/indexer.test.ts — Complete Vitest suite containing 21 tests covering unit mappings, mock mode sync, repeated syncs, cursor resumption, live mode queries, and error handling.

Acceptance Criteria Met

• Indexer has mock mode (mock: true or configuration-driven)
• Indexer stores last processed cursor/checkpoint (StellarIndexerCursor model)
• Indexer handles duplicate events safely (MongoDB 11000 duplicate keys are caught and logged, returning success counts)
• Indexer maps event types to application-level records (mapEventToChainMoveRecord)
• Tests use mocked Stellar responses (Vitest mocks and vi.stubGlobal for fetch)

Screenshot

[Obiajulu-gif]

Great work on implementing the Stellar event indexing service and providing a comprehensive test suite. I especially appreciate the attention to idempotency, cursor checkpointing, and the coverage around mock and live modes.

Thanks for the solid contribution! 🚀

[Obiajulu-gif]

Resolve your CI Errors

[Obiajulu-gif]

@devfoma that a great work you did there