feat(netutil): add local and private host detection helper

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/infrastructure/netutil/local_network.go

@@ -0,0 +1,32 @@
+package netutil
+
+import (
+	"net"
+	"strings"
+)
+
+func IsLocalOrPrivateHost(host string) bool {
+	host = strings.ToLower(strings.TrimSpace(host))
+	if host == "" {
+		return false
+	}
+	if host == "localhost" || strings.HasSuffix(host, ".localhost") || host == "host.docker.internal" {
+		return true
+	}
+	ip := net.ParseIP(host)
+	return ip != nil && IsLocalOrPrivateIP(ip)
+}
+
+func IsLocalOrPrivateIP(ip net.IP) bool {
+	if ip == nil {
+		return false
+	}
+	if ip.IsLoopback() || ip.IsPrivate() || ip.IsUnspecified() || ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() {
+		return true
+	}
+	ip4 := ip.To4()
+	if ip4 == nil {
+		return false
+	}
+	return ip4[0] == 100 && ip4[1] >= 64 && ip4[1] <= 127
+}