Integration test: indexer against a Postgres fixture

[iexwr]

Context

The indexer has no automated test, so regressions in event mapping go unnoticed.

Acceptance criteria

• Add a vitest suite that spins up Postgres (testcontainers or a docker-compose service in CI), seeds a Cursor, feeds a fixture Soroban event, and asserts the resulting PaymentRequest row.
• Runs in CI (extend .github/workflows/ci.yml with a Postgres service).

Technical notes

• Extract the mapping logic into a pure function applyEvent(event) -> Prisma.Write so it is testable without a live RPC.

Testing

• New test file src/indexer.test.ts.