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.

examples/sendmsg-email.sh

@@ -0,0 +1,36 @@
+#!/bin/bash
+# Example: Send email via notification/sendMsg endpoint
+
+BASE_URL="${1:-https://api.riotpiao.com}"
+AUTH_TOKEN="${2:-}"  # Optional JWT token if auth required
+
+PAYLOAD=$(cat <<'EOF'
+{
+  "format": "smtp",
+  "title": "System Alert",
+  "message": "CPU usage exceeded 90% threshold",
+  "priority": 7,
+  "extras": {
+    "to_email": "[email protected]",
+    "cc": "[email protected]"
+  }
+}
+EOF
+)
+
+if [ -n "$AUTH_TOKEN" ]; then
+  curl -X POST "$BASE_URL" \
+    -H "X-Service: notification" \
+    -H "X-Resource: sendMsg" \
+    -H "Content-Type: application/json" \
+    -H "Authorization: Bearer $AUTH_TOKEN" \
+    -d "$PAYLOAD"
+else
+  curl -X POST "$BASE_URL" \
+    -H "X-Service: notification" \
+    -H "X-Resource: sendMsg" \
+    -H "Content-Type: application/json" \
+    -d "$PAYLOAD"
+fi
+
+echo ""