Add fake payment API

[loganoe]

Summary

/claim #1

• add in-memory fake payment records with pending/completed/cancelled statuses
• add Winterspec routes for send, list, get, complete, and cancel payment flows
• support idempotency keys so repeated send requests return the original payment
• add route coverage for listing/filtering, fetching, idempotent sends, completion, and cancellation
• document the fake payment endpoints in the README

Transparency: AI-assisted with Codex; I reviewed the diff and verification results.

Verification

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