test(payouts): add route integration coverage

[h1065153539-create]

Summary

• add focused supertest integration coverage for POST /api/payouts
• cover happy-path settlement creation, optional defaults, zod validation envelopes, missing/insufficient auth, idempotency replay/mismatch, duplicate transaction upserts, and persistence failure mapping
• import the payout route schema directly from schemas/payout.js so payouts tests do not depend on unrelated OpenAPI schema side effects from the barrel export

Closes #457

Validation

• npm test -- payouts passed: 2 files, 13 tests
• npm run test:coverage -- payouts executed the focused suite successfully, then failed only because the repo's global coverage thresholds are applied to all src/**/*.ts and src/sdk/** when running a subset
• file-level coverage from coverage/coverage-final.json for src/routes/payouts.ts: statements 100%, functions 100%, branches 100%
• git diff --check passed

Existing upstream failures observed

• npm run lint currently fails before linting because eslint.config.js imports missing src/observability/eslint-plugin-logger-schema.js while only the .ts source exists.
• npm run build is currently blocked by broad pre-existing TypeScript errors outside this change, including duplicate config keys, missing generated gRPC modules, bond.ts OpenAPI type augmentation, SDK constructor type drift, and other unrelated files.

Notes

• The current payouts route only exposes create/upsert behavior at POST /api/payouts; there is no separate status-transition endpoint to exercise not-found transitions. The new tests cover the route's available conflict path through idempotency key mismatch and duplicate transaction-hash upsert semantics.