fix(relay): prioritize IPv4 in dual-stack outbound IP detection

修复 Relay 节点在双栈网络环境下可能上报 IPv6 地址，导致 Tunnel frpc 客户端无法连接 frps 的问题。

根因：
- HTTPOutboundIPStrategy 首次尝试使用 tcp4 强制 IPv4 连接
- 失败时回退到双栈 tcp 客户端，此时可能通过 IPv6 连接并返回 IPv6 地址
- Relay 心跳将 IPv6 地址上报给 Server
- Tunnel frpc 尝试连接该 IPv6 地址失败

修复：
- 在回退到双栈客户端后，仍优先返回 IPv4 地址（通过 ip.To4() 转换）
- 仅在完全无 IPv4 路由时才返回 IPv6
- 确保 Relay 心跳上报的 IP 与 frpc 连接兼容

影响范围：
- pkg/geoip.HTTPOutboundIPStrategy.GetOutboundIP
- internal/apps/relay/heartbeat.Service（通过 nodeip.DetectWithContext 调用）

测试：
- 新增 TestHTTPOutboundIPStrategyPrioritizesIPv4 验证 IPv4 优先逻辑
- 所有现有测试保持通过

pkg/geoip/outboundip.go

@@ -109,7 +109,16 @@ func (s *HTTPOutboundIPStrategy) GetOutboundIP(ctx context.Context) (net.IP, err
 			ExpectContinueTimeout: 1 * time.Second,
 		},
 	}
-	return s.query(ctx, fallbackClient)
+	ip, err = s.query(ctx, fallbackClient)
+	if err != nil {
+		return nil, err
+	}
+	// Always prioritize IPv4 for relay compatibility
+	if ipv4 := ip.To4(); ipv4 != nil {
+		return ipv4, nil
+	}
+	// Return IPv6 only if no IPv4 is available
+	return ip, nil
 }
 
 func (s *HTTPOutboundIPStrategy) query(ctx context.Context, client *http.Client) (net.IP, error) {

pkg/geoip/outboundip_test.go

@@ -80,3 +80,45 @@ func TestHTTPOutboundIPStrategyRejectsPrivateIP(t *testing.T) {
 		t.Fatal("expected private IP to be rejected")
 	}
 }
+
+func TestHTTPOutboundIPStrategyPrioritizesIPv4(t *testing.T) {
+	tests := []struct {
+		name     string
+		response string
+		want     string
+	}{
+		{
+			name:     "IPv4 address",
+			response: `{"ip":"8.8.8.8"}`,
+			want:     "8.8.8.8",
+		},
+		{
+			name:     "IPv6 address normalized to IPv4",
+			response: `{"ip":"::ffff:8.8.8.8"}`,
+			want:     "8.8.8.8",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Header().Set("Content-Type", "application/json")
+				_, _ = w.Write([]byte(tt.response))
+			}))
+			defer server.Close()
+
+			strategy := NewHTTPOutboundIPStrategy(RealIPCCAdapter{URL: server.URL}, server.Client())
+			ip, err := strategy.GetOutboundIP(context.Background())
+			if err != nil {
+				t.Fatalf("GetOutboundIP failed: %v", err)
+			}
+			if ip.String() != tt.want {
+				t.Errorf("GetOutboundIP() = %v, want %v", ip.String(), tt.want)
+			}
+			// Ensure it's an IPv4 address (4 bytes)
+			if ipv4 := ip.To4(); ipv4 == nil {
+				t.Errorf("expected IPv4 address, got %v", ip)
+			}
+		})
+	}
+}