fix: 为 Anthropic Messages API 流式转发添加下游 keepalive ping

Anthropic Messages API 的流式转发路径（gateway_service.go）在上游长时间
无数据时（如 Opus extended thinking 阶段）不会向下游发送任何内容，导致
Cloudflare Tunnel 等代理因连接空闲而断开。

复用已有的 StreamKeepaliveInterval 配置（默认 10 秒），在 select 循环中
添加 keepalive 分支，定时发送 Anthropic 原生格式的 ping 事件保活，与
OpenAI 兼容路径的实现模式保持一致。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/gateway_service.go

@@ -5998,6 +5998,22 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 		intervalCh = intervalTicker.C
 	}
 
+	// 下游 keepalive：防止代理/Cloudflare Tunnel 因连接空闲而断开
+	keepaliveInterval := time.Duration(0)
+	if s.cfg != nil && s.cfg.Gateway.StreamKeepaliveInterval > 0 {
+		keepaliveInterval = time.Duration(s.cfg.Gateway.StreamKeepaliveInterval) * time.Second
+	}
+	var keepaliveTicker *time.Ticker
+	if keepaliveInterval > 0 {
+		keepaliveTicker = time.NewTicker(keepaliveInterval)
+		defer keepaliveTicker.Stop()
+	}
+	var keepaliveCh <-chan time.Time
+	if keepaliveTicker != nil {
+		keepaliveCh = keepaliveTicker.C
+	}
+	lastDataAt := time.Now()
+
 	// 仅发送一次错误事件，避免多次写入导致协议混乱（写失败时尽力通知客户端）
 	errorEventSent := false
 	sendErrorEvent := func(reason string) {
@@ -6187,6 +6203,7 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 							break
 						}
 						flusher.Flush()
+						lastDataAt = time.Now()
 					}
 					if data != "" {
 						if firstTokenMs == nil && data != "[DONE]" {
@@ -6220,6 +6237,22 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 			}
 			sendErrorEvent("stream_timeout")
 			return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream data interval timeout")
+
+		case <-keepaliveCh:
+			if clientDisconnected {
+				continue
+			}
+			if time.Since(lastDataAt) < keepaliveInterval {
+				continue
+			}
+			// SSE ping 事件：Anthropic 原生格式，客户端会正确处理，
+			// 同时保持连接活跃防止 Cloudflare Tunnel 等代理断开
+			if _, werr := fmt.Fprint(w, "event: ping\ndata: {\"type\": \"ping\"}\n\n"); werr != nil {
+				clientDisconnected = true
+				logger.LegacyPrintf("service.gateway", "Client disconnected during keepalive ping, continuing to drain upstream for billing")
+				continue
+			}
+			flusher.Flush()
 		}
 	}