ci: checkout tagged source for manual releases

.woodpecker/release-backend.yml

@@ -16,6 +16,16 @@ clone:
       lfs: false
 
 steps:
+  checkout-release-source:
+    image: bash
+    commands:
+      - set -euo pipefail
+      - release_tag="${CI_COMMIT_TAG:-${RELEASE_TAG:-}}"
+      - test -n "$release_tag"
+      - node scripts/check-api-contract.mjs "$release_tag"
+      - git fetch --force origin "refs/tags/$release_tag:refs/tags/$release_tag"
+      - git checkout --detach "refs/tags/$release_tag"
+
   release-checks:
     image: bash
     commands: