Fix build: fetch latest upstream webhook release tarball

Upstream stopped shipping bare binaries; 2.8.2 URL 404s. Resolve the latest
release at build time so scheduled rebuilds track upstream.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SNMztZDM1qUGnixHeevkuw

.gitlab-ci.yml

@@ -9,7 +9,7 @@ build:
   before_script:
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" "$CI_REGISTRY"
   script:
-    - docker build -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA" -t "$CI_REGISTRY_IMAGE:latest" .
+    - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA" -t "$CI_REGISTRY_IMAGE:latest" .
     - docker push --all-tags "$CI_REGISTRY_IMAGE"
   rules:
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH