Add hello health endpoint

[daxia778]

Summary

• add a public GET /hello route that returns { "ok": true }
• add coverage at the app layer so the route is verified before the frontend catch-all

Closes #118

Validation

• pnpm --filter @riona/api test -- app.test.ts --runInBand --forceExit
• pnpm --filter @riona/api test -- routes/api.test.ts --runInBand --forceExit
• pnpm --filter @riona/api test -- --runInBand --forceExit
• pnpm --filter @riona/api typecheck
• pnpm --filter @riona/api lint
• git diff --check -- apps/api/src/app.ts apps/api/src/app.test.ts

Payout

Payment preference after acceptance: USDC on Base to 0xB54Ec1F0905186553FaB1763409e1CC127b874F9.