CVE-2026-41189: FreeScout has assigned-only visibility bypass that allows editing hidden customer-authored threads

FreeScout is a free self-hosted help desk and shared mailbox. Prior to version 1.8.215, customer-thread editing is authorized through `ThreadPolicy::edit()`, which checks mailbox access but does not apply the assigned-only restriction from `ConversationPolicy`. A user who cannot view a conversation can still load and edit customer-authored threads inside it. Version 1.8.215 fixes the vulnerability.