feat: add Stellar-signature-based JWT authentication guard

[therealsaitama0]

Closes #6

Implements Stellar-signature-based JWT authentication for protected write endpoints.

• Adds GET /api/auth/nonce endpoint for nonce generation
• Adds POST /api/auth/verify endpoint for Stellar signature verification and JWT issuance
• Applies JwtAuthGuard to POST /api/groups, POST /api/loans, and PATCH /api/loans/:id/status
• Public read endpoints remain unguarded: GET /api/groups, GET /api/stats, GET /api/loans, GET /api/groups/:id
• Swagger auth configured via existing @ApiBearerAuth() in DocumentBuilder

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[therealsaitama0]

Payment ETH address for this bounty: 0x5e1040927a1E28D740f92De27a3d493b81682D88
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[therealsaitama0]

Auth implementation summary:

• Nonce endpoint: GET /api/auth/nonce?address=<stellar_address>
• Verify endpoint: POST /api/auth/verify { address, signedNonce }
• Storage: in-memory Map keyed by lower-cased address with 5-minute TTL
• Verification: Keypair.verify from @stellar/stellar-sdk
• JWT: issued via @nestjs/jwt and used by JwtAuthGuard (passport-jwt strategy)
• Guards applied:
  • POST /api/groups
  • POST /api/loans
  • PATCH /api/loans/:id/status
• Public reads stay unguarded: GET /api/groups, GET /api/stats, GET /api/loans, GET /api/groups/:id
• Swagger bearer auth is already configured in main.ts and @ApiBearerAuth() is applied on protected endpoints
• Build verified: npm run build

Bounty payment ETH address: 0x5e1040927a1E28D740f92De27a3d493b81682D88
Paypal: https://paypal.me/therealsaitama