fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/contrib/python/examples/ldns-zone.py

@@ -0,0 +1,15 @@
+#!/usr/bin/python
+import ldns
+
+#Read zone from file
+zone = ldns.ldns_zone.new_frm_fp(open("zone.txt","r"), None, 0, ldns.LDNS_RR_CLASS_IN)
+print zone
+
+print "SOA:", zone.soa()
+for r in zone.rrs().rrs():
+   print "RR:", r
+
+
+zone = ldns.ldns_zone()
+#print zone
+