Every /api/xmpp/health fetch ran a real LLM call (22k token system prompt
each). Dashboard refreshes every 10s -> thousands of paid LLM calls/day,
plus 38s latency hanging the health tab on '加载中...'.
LLM health is now derived from the gateway's own agent.log (zero cost,
more accurate than synthetic ping — real traffic results):
- last 'API call #N latency=Xs' -> ok
- last 'API call failed ... HTTP 429...' -> error with summary
- health() runtime 38s -> 0.1s; endpoint 38s -> 0.097s
