feat: show live build version in web UI

- /api/version returns version, commitSha, buildDate, trashRetentionDays
- CI passes COMMIT_SHA (full), BUILD_DATE and VERSION (git tag) as
  docker build args; Dockerfile bakes them as env
- sidebar footer shows version/sha linked to the Gitea commit
- admin System tab shows version, commit link, build date, trash retention

.gitea/workflows/build.yaml

@@ -27,8 +27,14 @@ jobs:
       - name: Build and push
         run: |
           SHA=$(git rev-parse --short HEAD)
+          FULL_SHA=$(git rev-parse HEAD)
           IMAGE="git.kubebase.de/${{ gitea.repository }}"
-          docker build --no-cache -t "${IMAGE}:sha-${SHA}" -t "${IMAGE}:latest" .
+          if [ "${{ gitea.ref_type }}" = "tag" ]; then VERSION="${{ gitea.ref_name }}"; else VERSION=""; fi
+          docker build --no-cache \
+            --build-arg COMMIT_SHA="$FULL_SHA" \
+            --build-arg BUILD_DATE="$(date -u +"%Y-%m-%dT%H:%M:%SZ")" \
+            --build-arg VERSION="$VERSION" \
+            -t "${IMAGE}:sha-${SHA}" -t "${IMAGE}:latest" .
           docker push "${IMAGE}:sha-${SHA}"
           docker push "${IMAGE}:latest"