feat(backend): implement SIWS message verification (#108)

[omonxooo-commits]

Summary

Closes #108

Implements Sign-In With Stellar (SIWS) message verification on the backend.

Changes

• services/siws.rs — nonce generation, canonical SIWS message builder, Stellar strkey → ed25519 pubkey decode, verify_strict signature check
• routes/auth.rs — POST /api/v1/auth/nonce and POST /api/v1/auth/verify with one-time nonce consumption (replay-safe)
• db.rs — NonceStore (Arc<Mutex<HashMap>>) added to AppState
• routes/mod.rs — /auth router registered under /api/v1
• Cargo.toml — added rand, hex, base32 deps
• .env.example — added APP_DOMAIN and SESSION_SECRET

Auth flow

POST /api/v1/auth/nonce  { address }              → { nonce, issued_at }
POST /api/v1/auth/verify { address, signature }   → { token }

Security

• Nonces consumed on first use — no replay
• verify_strict rejects malleable signatures
• Domain bound in signed message — prevents cross-domain phishing