docs: add production deployment guide with systemd service and Caddy basic auth

[jelloeater-agent]

Adds a "Running as a Service" section to the README covering:

• systemd service — proper unit file with Restart=on-failure, configurable port via PORT=6970
• Caddy reverse proxy — basic auth on the default :6969 port, proxying to the internal port
• bcrypt gotcha — Caddy 2.6+ requires bcrypt-hashed passwords, not plaintext. Documents the env-file + systemd drop-in pattern to avoid dollar escaping issues in the Caddyfile.