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/smtp-secrets.example.yaml

@@ -0,0 +1,29 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: smtp-credentials
+  namespace: api
+  labels:
+    app: api-gateway
+    component: notification
+type: Opaque
+data:
+  host: <base64-encoded SMTP hostname>
+  port: <base64-encoded SMTP port, e.g., "587">
+  from: <base64-encoded sender email>
+  user: <base64-encoded SMTP username>
+  password: <base64-encoded SMTP password>
+
+# To create from plaintext:
+# kubectl create secret generic smtp-credentials \
+#   --from-literal=host=mail.example.com \
+#   --from-literal=port=587 \
+#   [email protected] \
+#   --from-literal=user=smtp-user \
+#   --from-literal=password=smtp-pass \
+#   -n api \
+#   -o yaml > smtp-secrets.yaml
+#
+# Then encrypt with SOPS:
+# sops -e smtp-secrets.yaml > smtp-secrets.enc.yaml
+# rm smtp-secrets.yaml