CVE-2026-45742: Gotenberg has a Race Condition via Multipart `downloadFrom` Handling

Gotenberg is vulnerable to a remote denial of service in multipart downloadFrom handling.

A multipart request containing multiple downloadFrom entries causes concurrent goroutines to write to shared maps without synchronization. This can terminate the process with fatal error: concurrent map writes.

In the default configuration, downloadFrom is enabled and authentication is disabled, so an exposed instance can be crashed by an unauthenticated remote attacker.