Add Stellar JWT authentication guard

[bingmokaka]

Summary

• add Stellar wallet nonce and signature verification endpoints under /api/auth
• issue JWTs after successful nonce verification and validate Bearer tokens with passport-jwt
• protect POST /api/groups, POST /api/loans, and PATCH /api/loans/:id/status while leaving read endpoints public
• add auth service unit coverage and fix existing TypeScript build issues surfaced by current dependencies

Testing

• npm test -- --runInBand
• npm run build

Closes #6