test: integration tests for full stream → delivery pipeline`

[cokehill]

Summary

Unit tests cover individual functions, but we need integration tests that simulate a real
Stellar testnet event flowing through the full pipeline to a webhook receiver.

Acceptance criteria

• Test harness spins up in-memory SQLite and the Express app
• Mock Horizon SSE endpoint using nock or msw
• Assert that a mocked payment event creates a delivery row with status = delivered
• Assert webhook receiver (local http server in test) receives correct payload
• Assert HMAC signature validates against shared secret
• Assert retry fires after simulated 500 response from receiver
• CI runs these tests via npm test without network access

Tech

Jest + supertest + nock.
Tests live in src/__tests__/integration/.