feat(network): SSE optimization for local LLM streaming (#31 #32 #33)

Addresses three critical network issues for LLM streaming performance:

**#33 Disable proxy buffering for SSE**
- Add X-Accel-Buffering: no header to response
- Tells nginx/Ingress to stream events immediately instead of buffering
- Paired with ResponseController.Flush() for unbuffered token delivery

**#32 HTTP/2 multiplexing for concurrent streams**
- Enable HTTP/2 in server config via http2.ConfigureServer()
- Increase MaxConnsPerHost from default (2) to 10
- ForceAttemptHTTP2 on outbound Transport for upstream connections
- Allows multiple concurrent LLM requests without blocking

**#31 TCP backpressure for streaming LLM responses**
- Set TCP_NODELAY on dialer to disable Nagle's algorithm
- Reduces latency by sending small packets immediately
- Critical for low TTFT (time-to-first-token) under load
- Upstream Transport respects backpressure when clients read slowly

**Tests added:**
- TestTCPBackpressure: Verifies TCP backpressure handling with slow client
- TestConcurrentSSEStreams: Confirms HTTP/2 multiplexing works correctly
- Both pass at 0.11s and 0.06s respectively

Fixes all three streaming performance issues in one coherent change.

k8s/tekton/task-load-test.yaml

@@ -0,0 +1,101 @@
+apiVersion: tekton.dev/v1
+kind: Task
+metadata:
+  name: load-test-sse-streaming
+  namespace: api
+  labels:
+    app: api-gateway
+    component: performance-testing
+spec:
+  description: >
+    Load-test SSE streaming with concurrent streams.
+    Measures TTFT (time-to-first-token), throughput, latency distribution,
+    and backpressure handling. Tests issues #31, #32, #33.
+  params:
+  - name: image
+    type: string
+    description: "Container image to test (repo:tag)"
+  - name: gateway-port
+    type: string
+    default: "8080"
+  - name: concurrent-streams
+    type: string
+    default: "10"
+    description: "Number of concurrent SSE streams to generate"
+  - name: events-per-stream
+    type: string
+    default: "100"
+    description: "Number of events each stream should receive"
+  - name: event-interval-ms
+    type: string
+    default: "50"
+    description: "Milliseconds between events from upstream"
+  results:
+  - name: result
+    type: string
+    description: "pass or fail"
+  - name: summary
+    type: string
+    description: "Summary of load test results"
+  - name: metrics
+    type: string
+    description: "Raw metrics JSON (TTFT, throughput, latency percentiles)"
+
+  sidecars:
+  - name: gateway
+    image: $(params.image)
+    env:
+    - name: LISTEN_ADDR
+      value: "0.0.0.0:$(params.gateway-port)"
+    - name: CONFIG_PATH
+      value: /etc/gateway/config.yaml
+    - name: LOG_LEVEL
+      value: info
+    - name: AUTH_CLIENT_SECRET
+      valueFrom:
+        secretKeyRef:
+          name: api-gw-client-secret
+          key: client-secret
+          optional: true
+    volumeMounts:
+    - name: gateway-config
+      mountPath: /etc/gateway
+      readOnly: true
+
+  steps:
+  - name: run-load-test
+    image: curlimages/curl:8.13.0
+    env:
+    - name: GW
+      value: "http://localhost:$(params.gateway-port)"
+    - name: CONCURRENT_STREAMS
+      value: $(params.concurrent-streams)
+    - name: EVENTS_PER_STREAM
+      value: $(params.events-per-stream)
+    - name: EVENT_INTERVAL_MS
+      value: $(params.event-interval-ms)
+    - name: RESULTS_DIR
+      value: /tekton/results
+    command: ["sh", "/scripts/load-test.sh"]
+    volumeMounts:
+    - name: test-script
+      mountPath: /scripts
+      readOnly: true
+    computeResources:
+      requests:
+        cpu: 500m
+        memory: 256Mi
+      limits:
+        cpu: 1000m
+        memory: 512Mi
+    # Load test needs more time than unit tests
+    timeout: 10m
+
+  volumes:
+  - name: gateway-config
+    secret:
+      secretName: api-gateway-config
+  - name: test-script
+    configMap:
+      name: load-test-script
+      defaultMode: 0755