CVE-2026-40937: RustFS: Missing admin authorization on notification target endpoints allows unauthenticated configuration of event webhooks

All four notification target admin API endpoints in rustfs/src/admin/handlers/event.rs use a check_permissions helper that validates authentication only (access key + session token), without performing any admin-action authorization via validate_admin_request. Every other admin handler in the codebase correctly calls validate_admin_request with a specific AdminAction. This is the only admin handler file that skips authorization.

A non-admin user can overwrite a shared admin-defined notification target by name, causing subsequent bucket events to be delivered to an attacker-controlled endpoint. This enables cross-user event interception and audit evasion.