Use system DNS resolver instead of custom infra-dns

infra-dns container has UDP connectivity issues to upstream DNS.
System resolver works (proven via wget test).

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

crawler.go

@@ -145,26 +145,11 @@ func (c *Crawler) StartMaintenanceLoop() {
 	}
 }
 
-// dnsResolver uses local caching DNS (infra-dns) with fallback to system
-var dnsResolver = &net.Resolver{
-	PreferGo: true,
-	Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
-		d := net.Dialer{Timeout: 2 * time.Second}
-		// Try local caching DNS first (CoreDNS on proxy network)
-		conn, err := d.DialContext(ctx, "udp", "infra-dns:53")
-		if err == nil {
-			return conn, nil
-		}
-		// Fallback to system DNS
-		return d.DialContext(ctx, network, address)
-	},
-}
-
 // domainCheck performs a DNS lookup to check if a domain resolves
 func (c *Crawler) domainCheck(host string) error {
-	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	_, err := dnsResolver.LookupHost(ctx, host)
+	_, err := net.DefaultResolver.LookupHost(ctx, host)
 	return err
 }