feat: Add JWT authentication to admin endpoints

[Ishant5436]

Summary

• Adds Stellar-signature-based auth (GET /api/auth/nonce → sign nonce → POST /api/auth/verify → JWT) per the flow described in the issue
• Applies JwtAuthGuard to the write endpoints: POST /groups, POST /loans, PATCH /loans/:id/status
• Read endpoints (GET /groups, GET /loans, etc.) remain public
• Swagger @ApiBearerAuth() wired on the guarded routes
• e2e test covers the full nonce → verify → protected-route flow

Closes #6

Test plan

• GET /api/auth/nonce?address=<stellar_address> returns a nonce
• Signing the nonce with the matching keypair and posting to /api/auth/verify returns a JWT
• JWT is accepted as Authorization: Bearer <token> on protected routes
• Unauthenticated requests to guarded routes are rejected
• npm run test:e2e passes locally