feat(backend): SIWS wallet auth with JWT + Socket.IO middleware

[codebestia]

Summary

• Installed @stellar/stellar-sdk and jsonwebtoken for Stellar signature verification and session tokens
• POST /auth/challenge — generates a signed message + nonce (5-min TTL) for a given wallet address
• POST /auth/verify — verifies Stellar keypair signature, upserts user + wallet in DB, returns a JWT
• src/lib/jwt.ts — typed signToken / verifyToken helpers (7-day expiry)
• src/lib/nonce.ts — in-memory nonce store with 5-minute expiry and single-use consumption
• src/middleware/auth.ts — requireAuth Express middleware that reads Authorization: Bearer <token>
• src/middleware/socketAuth.ts — Socket.IO middleware that reads socket.handshake.auth.token
• Added GET /me as a protected route example
• Added JWT_SECRET to .env.example