Feat: Stellar client support

[CeceOs92]

Summary

Added a server-only Stellar client helper for Horizon and Soroban RPC, with validated Testnet/Mainnet selection and network passphrases. Added tests and evidence screenshots.

Area changed

Check all that apply:

• Frontend / UI
• Backend / API routes
• Auth / Privy
• Payments / Paystack
• Email / Resend
• Stellar / Soroban
• MongoDB models
• Documentation
• Tests
• Security

Contributor safety checklist

• I did not commit .env.local or real secrets.
• I did not expose server-only variables in client-side code.
• I used mock mode or my own sandbox/test credentials.
• I did not add deployment steps for contributor PRs.
• I did not add maintainer-only credentials.

Testing

• npm run lint ✅
• Targeted Stellar tests: 10 passed ✅
• npm run build ✅
• Full npm test has one pre-existing Mongo hook timeout in __tests__/models/User.test.ts.
• npm run typecheck --if-present has pre-existing application type errors unrelated to this change.

Closes #3