CVE-2026-106498: Backstage: Improper URL validation in catalog entity placeholder resolution

### Impact An authenticated Backstage user could craft a catalog entity with placeholder directives that reference resources outside the entity's source repository. Under certain configurations, this could allow access to data not intended to be available to the user. ### Patches Patched in `@backstage/plugin-catalog-backend` version `3.9.1` ### Workarounds If you're not able to update immediately: - Limit the scope of integration credentials (e.g., GitHub tokens) to only the repositories that Backstage needs to access.