Add fake payment lifecycle API

[dustinvali]

Summary

• add fake payment records to the in-memory zod/zustand database
• add Winterspec routes for send, list, get, complete, and cancel payment flows
• support idempotency keys so retries do not create duplicate fake payments
• add list filters for recipient, status, owner, repo, and issue number
• add focused route tests for payment creation, lookup, idempotency, filtering, completion, and cancelation

/claim #1

Tests

• bunx biome check lib/db/schema.ts lib/db/db-client.ts routes/payments/send.ts routes/payments/list.ts routes/payments/get.ts routes/payments/complete.ts routes/payments/cancel.ts tests/routes/payments.test.ts
• bun test
• bun run build
• git diff --check