Add fake payment API

[vegapunk9527]

Summary

• add an in-memory payment schema and payment store helpers
• add POST /payments/send, GET /payments/list, and POST /payments/complete
• cover payment creation, idempotent resend, filtered listing, and completion in route tests

Testing

• npm exec --yes bun -- test
• npx biome check lib/db/schema.ts lib/db/db-client.ts routes/payments/send.ts routes/payments/list.ts routes/payments/complete.ts tests/routes/payments.test.ts
• npm run build
• npx tsc --noEmit

/claim #1