{blog.slug} used the prefix as both the FK field name and the table name, so
apps with prefixed tables (cms_blog) silently lost those purge paths. The
joined table now comes from the <prefix>Id field's possible-value source,
falling back to the bare prefix. A missing joined table is logged once.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>