fix: add gettext to cli-build stage for msgfmt locale compilation

share/GNUmakefile compiles .po -> .mo files via msgfmt (same pattern
as the engine build stage)

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

Dockerfile.backend

@@ -118,6 +118,7 @@ RUN apk add --no-cache \
 FROM engine AS cli-build
 
 RUN apk add --no-cache \
+    gettext \
     make \
     perl-app-cpanminus \
     perl-json-xs \