feat: implement node registration and heartbeat service

[Tonyfudecai]

Description

Implement the core Fleet Manager service: node registration and heartbeat monitoring.

Requirements

• POST /fleet/register: accept node_id, capabilities, location
• POST /fleet/heartbeat: update last_seen, status, metrics
• In-memory node registry (upgrade to DB later)
• Mark nodes as offline after heartbeat timeout
• List active/offline nodes

Skills

Go or TypeScript, HTTP server