feat: add public health check endpoint

[pdtkts]

Summary

• Add GET /health endpoint (no auth required)
• Returns backend_running (bool) and has_active_tokens (bool)
• Useful for uptime monitoring tools (UptimeRobot, Kuma, Docker healthcheck, etc.)

Response example

{
  "backend_running": true,
  "has_active_tokens": true
}

Details

• If DB connection fails, gracefully returns has_active_tokens: false instead of crashing
• Reuses existing get_dashboard_stats() query
• No breaking changes

Test plan

• curl http://localhost:38000/health returns correct JSON
• Docker build & run successfully