Add fake payment API

[lasgg]

Summary

• add a fake payment lifecycle API with send/list/get/complete/cancel/fail routes
• persist payment records in the existing in-memory zod/zustand store
• support retry-safe sends with optional idempotency_key and list filters for recipient, repository, and status
• document the payment endpoints in the README

Verification

• npx --yes bun test
• npx 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 tsc --noEmit
• git diff --check

/claim #1