Add fake payment API

[masuda-so]

/claim #1

Summary

• add an in-memory fake payment model with bounty metadata and lifecycle timestamps
• add send/list/get/complete/cancel/fail payment routes
• support idempotency keys so retries reuse the original fake payment
• document the fake payment endpoints in README

Verification

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

AI-assisted with Codex; I reviewed the diff and kept the change scoped.

[masuda-so]

Hi! Just a quick follow-up on this bounty PR.

I kept the implementation scoped to a fake payment lifecycle API, with idempotency-key support, route tests, README docs, and local verification passing:

• bun test
• tsc --noEmit
• biome check
• bun run build
• git diff --check

Happy to adjust the API shape if you prefer a smaller or different surface.