• Add jsonwebtoken crate for HS256 JWT generation and validation
• Create JWT service with token generation/validation methods
• Implement JWT middleware that validates Bearer tokens
• Maintain backward compatibility with existing SIWE signature auth
• Add POST /auth/login endpoint that returns JWT token after signature verification
• Add JWT_SECRET and JWT_EXPIRATION environment variables
• Update auth flow: JWT tokens can be used instead of signing every request

The auth middleware now supports dual authentication:

1. JWT Bearer token validation (faster, no signature verification needed)
2. SIWE signature verification (existing flow, still works)

This allows clients to authenticate once, get a JWT, and use it for subsequent requests without having to sign every message.

Closes #106