Add /status endpoint to worker

[iarturo]

The worker (worker/index.js) has no way to check if it's
running and how far behind it is.

Tasks:

• Add a simple HTTP server to the worker (use native http module)
• GET /status returns: last_block, current_block, lag, uptime
• Document the endpoint in README

Acceptance criteria:

• curl http://localhost:3001/status returns JSON
• lag field shows blocks behind current chain