Switch back to infra-dns for DNS lookups

infra-dns now configured with Charter DNS servers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crawler.go

@@ -145,11 +145,20 @@ func (c *Crawler) StartMaintenanceLoop() {
 	}
 }
 
+// dnsResolver uses local caching DNS (infra-dns)
+var dnsResolver = &net.Resolver{
+	PreferGo: true,
+	Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+		d := net.Dialer{Timeout: 5 * time.Second}
+		return d.DialContext(ctx, "udp", "infra-dns:53")
+	},
+}
+
 // domainCheck performs a DNS lookup to check if a domain resolves
 func (c *Crawler) domainCheck(host string) error {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	_, err := net.DefaultResolver.LookupHost(ctx, host)
+	_, err := dnsResolver.LookupHost(ctx, host)
 	return err
 }