Add fake payment API

[kpoxo6op]

Summary

• add a Winterspec-native fake payments API with send, list, get, and complete routes
• store fake payment records in the existing in-memory database
• add idempotency-key replay protection so retrying /payments/send does not create duplicate fake transfers
• document the payment routes in the README

/claim #1

Verification

• npx --yes bun test
• npx --yes bun test tests/routes/payments.test.ts
• npx --yes bun x biome check lib/db/schema.ts lib/db/db-client.ts routes/payments tests/routes/payments.test.ts README.md
• npx --yes bun run build
• npx --yes tsc --noEmit
• git diff --check