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>

Dockerfile.backend

@@ -7,8 +7,11 @@
 FROM alpine:3.22 AS ldns-build
 
 RUN apk add --no-cache \
+    autoconf \
+    automake \
     build-base \
     libidn2-dev \
+    libtool \
     make \
     openssl-dev \
     perl-app-cpanminus \