CVE-2026-42594: Gotenberg has an unauthenticated denial of service via echo.Context pool reuse in webhook async goroutine

The webhook middleware spawns a goroutine that holds a reference to the request’s echo.Context after the synchronous handler returns ErrAsyncProcess and Echo recycles the context back to its sync.Pool. When a concurrent request claims the recycled context, c.Reset() clears the store. If the webhook goroutine reaches hardTimeoutMiddleware at that moment, an unchecked type assertion on a nil store entry panics outside any recover() scope, crashing the Gotenberg process. Any anonymous caller reaches the webhook path (default webhook-deny-list filters only the webhook destination, not the submitter). A single-source stress of ~24 webhook requests plus ~60 GET /version requests crashes the process in about two seconds.