SGLang's /health takes ~1.001s, racing the 1s k8s probe timeout.
Now haproxy health-checks SGLang in the background (5s interval, 3s check timeout)
and responds to /health probes instantly: 200 if backend is up, 503 if not.
SGLang now runs on port+1, haproxy binds the original vLLM port.
haproxy serves a stub /metrics endpoint (200, empty body) and
passes all other traffic through to SGLang via raw TCP proxy.
