ci: pin registry login username to gitea-admin

REGISTRY_TOKEN was created under the gitea-admin user, so the
docker/helm registry username must match. Using github.actor
would fail for any other workflow-triggering user.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/rollback.yml

@@ -78,7 +78,7 @@ jobs:
         uses: docker/login-action@b45d80f862d83dbcd57f89517bcf500b2ab88fb2 # v4.0.0
         with:
           registry: git.farh.net
-          username: ${{ github.actor }}
+          username: gitea-admin
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Verify Docker image tag exists