Add fake payment lifecycle API

[DanielButler1]

Summary

• Add in-memory fake payment records with recipient, amount, currency, bounty metadata, status, timestamps, and optional idempotency keys.
• Add send, list, get, complete, and cancel payment routes.
• Make send retry-safe when the same idempotency key is reused.
• Add route tests for create/list/get/complete and cancel flows.

/claim #1

Verification

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