fix(proxy): address review findings on admission control (#64)

- tracing: responseWriter now forwards Flush and exposes Unwrap. Without it
  w.(http.Flusher) failed for every request passing the tracing middleware, so
  the parked-stream keep-alive never started: a parked streaming request wrote
  nothing for up to ADMISSION_MAX_PARK_SECONDS, which is exactly the client
  timeout the keep-alive was added to prevent.
- proxy: once the SSE status line is spent, ModifyResponse frames an upstream
  response >= 400 as an event: error instead of letting a raw error body land
  inside the stream, where a streaming client reads it as an empty completion
  and reports success. net/http keeps the first status, so the upstream status
  cannot be forwarded after the park has written one.
- engine_state: new Snapshot() returns the state and its tick channel under one
  lock. Get and Tick taken separately can miss the poll that lands between the
  two calls, parking a request for an extra interval.
- engine_state: observer_degraded now tracks current parsability rather than
  being raised once on the first poll and never cleared, which pinned it at 1
  after a single truncated response and never reported drift that appeared
  later. markDegraded only touches the exporter when the value changes.
- deployment: ADMISSION_MAX_INFLIGHT is set explicitly to 4, documenting the
  coupling to the engine's --max-num-seqs, instead of silently defaulting to
  VLLM_MAX_NUM_SEQS.

k8s/deployment.yaml

@@ -70,6 +70,12 @@ spec:
           value: "info"
         - name: ADMISSION_MAX_PARK_SECONDS
           value: "120"
+        # Must track the engine's --max-num-seqs (homelab: reasoning.yaml).
+        # The gauge this compares against is engine-wide, so this number is a
+        # ceiling on concurrent requests across all gateway replicas, not a
+        # per-pod slot count. Drift here is silent; startup logs the value.
+        - name: ADMISSION_MAX_INFLIGHT
+          value: "4"
         # Only backends that publish the vLLM scheduler gauges may be observed.
         # `reasoning` is the only vLLM engine; ornith/qwen-cpu are Ollama and
         # embeddings/rerank are TEI, so observing them would hold