Fediverse Embeds: Unauthenticated SSRF / open proxy via REST media-proxy endpoint

Fediverse Embeds embeds fediverse posts on WordPress sites. Prior to version 1.5.8, Fediverse Embeds registered an unauthenticated REST route ftf/media-proxy (includes/Media_Proxy.php) with permission_callback => __return_true that accepted a base64-encoded URL and forwarded it to wp_remote_get($url) without enforcing any allowlist. The plugin's source contained a comment block explicitly acknowledging that the request should be validated against allowed fediverse domains, but in 1.5.7 the validation only set a local $can_download_media flag that was never read. The full response body was echoed back to the caller, so this was a full-read SSRF / open proxy reachable by any anonymous visitor. This issue has been patched in version 1.5.8.