fix: add missing deps to backend-build stage

- gettext: msgfmt needed for share/GNUmakefile .po -> .mo compilation
- perl-libwww: provides LWP::UserAgent
- cpanm: Daemon::Control, JSON::RPC, JSON::Validator,
  Log::Any::Adapter::Dispatch, Plack::Middleware::ReverseProxy,
  Router::Simple::Declare — all required by Makefile.PL but absent

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

Dockerfile.backend

@@ -146,6 +146,7 @@ COPY --from=cli-build /usr/local/share/perl5/site_perl /usr/local/share/perl5/si
 FROM cli AS backend-build
 
 RUN apk add --no-cache \
+    gettext \
     make \
     curl \
     gcc \
@@ -163,6 +164,7 @@ RUN apk add --no-cache \
     perl-http-parser-xs \
     perl-mojolicious \
     perl-io-stringy \
+    perl-libwww \
     perl-log-any \
     perl-log-dispatch \
     perl-moose \
@@ -175,7 +177,13 @@ RUN apk add --no-cache \
     perl-try-tiny \
     perl-doc \
  && cpanm --notest --no-wget --from=https://cpan.metacpan.org/ \
-    Net::Statsd
+    Daemon::Control \
+    JSON::RPC \
+    JSON::Validator \
+    Log::Any::Adapter::Dispatch \
+    Net::Statsd \
+    Plack::Middleware::ReverseProxy \
+    Router::Simple::Declare
 
 COPY zonemaster-backend/ /src/backend/
 RUN cpanm --notest --no-wget /src/backend/