Add check_lfs_pointers CI job (tpo/web/blog#40036)

This job is added to fail CI when commits that include LFS-tracked files
are actually missing the LFS pointer. This can happen when files are
added via the GitLab web UI at the same time as a branch is created, see
tpo/web/blog#40036

lektor.yml

@@ -151,6 +151,23 @@ check_mangled_i18n:
       when: never
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
 
+# this check verifies that no pointers are missing for LFS-tracked files
+# the $CI_CHECK_LFS variable enables this job
+
+check_lfs_pointers:
+  stage: test
+  image: bookwork-slim
+  script:
+  before_script:
+    - !reference [.apt-init]
+  script:
+    - apt install -y git-lfs
+    - git lfs fsck --pointers
+  rules:
+    - if: $L10N_STAGING
+      when: never
+    - if: $CI_COMMIT_BRANCH && $CI_CHECK_LFS
+
 # this check verifies compares the PO files and determines
 # if there are any stale strings or new unstranslated strings
 # in which case it will trigger an alert in tpo/community/l10n