fix: use weighted random pick for load balancing instead of sequential iteration

server/internal/proxy/passthrough.go

@@ -163,11 +163,29 @@ func upstreamURL(ch *store.Channel, proto, path string) string {
 	return ch.UpstreamURL(proto, path)
 }
 
-// doProxy 通用代理（M5）：遍历候选渠道，按需转换；可安全重试的失败自动故障转移。
+// doProxy 通用代理（M5）：加权随机选一个候选渠道；失败自动故障转移。
 func (g *Gateway) doProxy(c *gin.Context, cands []channel.Candidate, clientProto string, body []byte, stream bool, sink *usageSink) {
+	if len(cands) == 0 {
+		apiError(c, http.StatusBadGateway, "upstream_error", "no available channel")
+		g.recordError(c, nil, nil, now(), "no_available_channel")
+		return
+	}
+	// 加权随机选择起始渠道
+	picked := g.ch.Pick(cands)
+	startIdx := 0
+	for i, cand := range cands {
+		if cand.Channel.ID == picked.ID {
+			startIdx = i
+			break
+		}
+	}
+
 	var lastStatus = http.StatusBadGateway
 	var lastBody = []byte("all upstream channels failed")
-	for _, cand := range cands {
+	// 从选中的渠道开始遍历，到末尾后再从头遍历到选中渠道之前
+	for offset := 0; offset < len(cands); offset++ {
+		idx := (startIdx + offset) % len(cands)
+		cand := cands[idx]
 		ch := cand.Channel
 		plan, err := prepareUpstream(ch, clientProto, body, cand.UpstreamModel)
 		if err != nil {