Per-board moderators and granular permissions

Permission model beyond org owner/admin/member. Let admins assign moderators to specific boards with scoped abilities: triage posts, edit tags, change statuses, moderate comments, manage support tickets. Useful for large workspaces and community-heavy projects.

Schema: project_permissions or board_roles. UI in settings/accounts. RLS updates required across posts/comments/tickets/docs. Later feature because permissions complexity touches everything.