/claim #1

Summary

• add in-memory fake payment records with sent/completed/canceled/failed statuses
• add send/list/get/complete/cancel payment endpoints with idempotency-key deduplication
• document payment endpoints and add route tests for sends, filters, retries, completion, and cancellation

Verification

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