fix: use 127.0.0.1 in healthcheck to avoid IPv6 issues

Alpine's wget resolves localhost to IPv6 [::1] first, but the
Node.js server binds only to 0.0.0.0 (IPv4). This causes the
healthcheck to fail with "Connection refused".

🤖 Generated with [Claude Code](https://claude.com/claude-code

)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>