feat: add Stellar reconciliation explorer

[ayaanoncrypto]

Closes #145

Summary

This PR adds a persistent Stellar transaction reconciliation and explorer module.

Implementation

• Adds invoice, Stellar transaction, and reconciliation audit entities with indexed lookup fields.
• • Adds idempotent payment ingestion for Horizon or webhook consumers.
• • Matches payments by destination account, asset, and optional memo or payment reference.
• • Tracks open, partial, paid, failed, matched, partial, and unmatched states.
• • Records every reconciliation decision, retry, duplicate event, and failure in an audit trail.
• • Adds testnet Horizon polling with paging-token continuation and retry-safe failure handling.
• • Adds lookup endpoints for transactions and invoices.
• • Adds manual invoice reconciliation and admin-only unmatched and audit endpoints.
• • Documents configuration, payloads, lifecycle states, and API routes.

Validation

• Focused Jest suite: 6 tests passed.
• • Targeted TypeScript compilation passed for the reconciliation service, DTOs, entities, and tests.
• • Prettier check passed.
• • Git diff check passed.
• • Commit is GPG signed and verified locally.