ci: tolerate legacy Backend release webhook [CI]

ci/publish-release-offer.sh

@@ -8,7 +8,18 @@ bash "$root/ci/release-manifest.sh" "$component" "$repository" "$digest" "$tag"
 if [[ -n "${RELEASE_API_URL:-}" && -n "${RELEASE_API_TOKEN:-}" ]]; then
   python3 -c 'import json,sys; print(json.dumps({"manifest": open(sys.argv[1]).read(), "signature": open(sys.argv[2]).read()}))' \
     "$outdir/manifest.json" "$outdir/manifest.sig" > "$outdir/payload.json"
-  curl --fail-with-body --silent --show-error --retry 3 -X POST "${RELEASE_API_URL%/}/api/admin/releases" \
-    -H "Authorization: Bearer $RELEASE_API_TOKEN" -H 'Content-Type: application/json' --data-binary "@$outdir/payload.json" >/dev/null
-  echo "registered the signed $component release with the panel ($channel)"
+  response="$outdir/response.json"
+  status="$(curl --silent --show-error --retry 3 -o "$response" -w '%{http_code}' -X POST "${RELEASE_API_URL%/}/api/admin/releases" \
+    -H "Authorization: Bearer $RELEASE_API_TOKEN" -H 'Content-Type: application/json' --data-binary "@$outdir/payload.json")"
+  if [[ "$status" != 2* ]]; then
+    if [[ "$component" == backend && "$status" == 400 ]] && grep -Fq 'manifest needs a known component' "$response"; then
+      echo "the panel does not support Backend update offers yet; bootstrap it once before future Backend offers"
+    else
+      echo "release offer registration failed (HTTP $status)" >&2
+      cat "$response" >&2
+      exit 1
+    fi
+  else
+    echo "registered the signed $component release with the panel ($channel)"
+  fi
 fi