InvoicePlane: Primary Administrator Privilege Downgrade via `Users::form()` (Missing Object-Level Authorization)

InvoicePlane is a self-hosted open source application for managing invoices, clients, and payments. In version 1.7.2, Users::form() performs no object-level authorization check on user_id = 1. A Secondary Administrator (user_type = 1, user_id != 1) can rewrite the Primary Administrator's user_type to 2 (Guest / read-only), destroying the root account's privilege and locking the legitimate owner out of the instance. At time of publication, there are no publicly available patches.