feat(proxy): engine-wide LLM admission control via vLLM gauges (#64)

The per-pod semaphore was not a cap: 3 replicas x 4 slots advertised 12
concurrent requests while the engine runs 4, and nothing coordinated the
replicas. Admission is now gated on the engine's own published gauges,
which is global by construction because every replica contends for the
same engine.

- new internal/proxy/engine_state.go: 2s scrape of the backend /metrics,
  hand-rolled exposition parse (no new dependency), unhealthy after 3
  consecutive failed polls, and a startup self-check that raises
  observer_degraded rather than falling back to fail-open silently
- gate: park while kv_cache_usage_perc >= 0.90 or running + waiting >=
  ADMISSION_MAX_INFLIGHT, bounded by ADMISSION_MAX_PARK_SECONDS, then
  503 + Retry-After: 5. Unknown state fails open on purpose
- streaming requests receive SSE keep-alive heartbeats while parked
- router: extract only the top-level model field instead of unmarshalling
  the whole body into a map; body cap from MAX_BODY_BYTES (1 MiB, the
  model path previously inherited the 100 MB default)
- GOMAXPROCS from the cgroup CPU quota: the container is limited to 500m
  while Go defaulted to the host core count and was CFS-throttled
- VLLM_MAX_QUEUE_DEPTH default 20 -> 8
- delete k8s/configmap.yaml: absent from kustomization.yaml, and the live
  ConfigMap of that name carries an unrelated key
- correct the llm-client NetworkPolicy comment; llm-serving-ingress now
  selects by namespace, not by that label

Observers run only for models named in ADMISSION_OBSERVE_MODELS.
reasoning is the only vLLM engine here; ornith and qwen-cpu are Ollama
and embeddings/rerank are TEI, none of which publish vllm:* series, so
observing them would hold observer_degraded at 1 permanently.

Companion change (engine flags + dashboard panels) is
riotpiao-poimen/homelab#51.

internal/observability/prometheus.go

@@ -48,6 +48,38 @@ func (p *PrometheusExporter) Export() string {
 	lines = append(lines, "# TYPE vllm_prefix_cache_hit_tokens_total counter")
 	lines = append(lines, fmt.Sprintf("vllm_prefix_cache_hit_tokens_total %d", p.metrics.GetPrefixCacheHitTokens()))
 
+	lines = append(lines, "# HELP admission_parked_total Requests parked waiting for engine admission")
+	lines = append(lines, "# TYPE admission_parked_total counter")
+	lines = append(lines, fmt.Sprintf("admission_parked_total %d", p.metrics.GetAdmissionParked()))
+
+	lines = append(lines, "# HELP admission_rejected_total Requests rejected by engine admission control")
+	lines = append(lines, "# TYPE admission_rejected_total counter")
+	rejected := p.metrics.GetAdmissionRejected()
+	reasons := make([]string, 0, len(rejected))
+	for reason := range rejected {
+		reasons = append(reasons, reason)
+	}
+	sort.Strings(reasons)
+	for _, reason := range reasons {
+		lines = append(lines, fmt.Sprintf(`admission_rejected_total{reason="%s"} %d`, reason, rejected[reason]))
+	}
+
+	lines = append(lines, "# HELP engine_state_stale_total Times the engine metrics feed went stale")
+	lines = append(lines, "# TYPE engine_state_stale_total counter")
+	lines = append(lines, fmt.Sprintf("engine_state_stale_total %d", p.metrics.GetEngineStateStale()))
+
+	lines = append(lines, "# HELP vllm_requests_running vLLM requests running as read by the gateway")
+	lines = append(lines, "# TYPE vllm_requests_running gauge")
+	lines = append(lines, fmt.Sprintf("vllm_requests_running %d", p.metrics.GetVLLMRequestsRunning()))
+
+	lines = append(lines, "# HELP vllm_kv_usage_perc vLLM KV cache usage fraction as read by the gateway")
+	lines = append(lines, "# TYPE vllm_kv_usage_perc gauge")
+	lines = append(lines, fmt.Sprintf("vllm_kv_usage_perc %g", p.metrics.GetVLLMKVUsagePerc()))
+
+	lines = append(lines, "# HELP observer_degraded Engine metrics observer startup self-check failed (1) or healthy (0)")
+	lines = append(lines, "# TYPE observer_degraded gauge")
+	lines = append(lines, fmt.Sprintf("observer_degraded %d", p.metrics.GetObserverDegraded()))
+
 	return strings.Join(lines, "\n") + "\n"
 }