Add fake payment lifecycle API

[newmattock]

Summary

• add in-memory fake payment records to the existing Zustand/zod database
• add Winterspec payment routes for send, list, get, complete, and cancel
• support retry-safe sends via idempotency_key
• document the endpoint flow in the README

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
• bun run build
• bunx tsc --noEmit
• git diff --check

/claim #1