Feature Request: Staff Permissions for external contributors

## Problem description

Currently, event contributors in Chapters cannot help in review or manage speaker submissions for an event unless they are onboarded and haven the organizer-level access.

This makes it difficult for organizers to collaborate with external contributors or volunteers who need to help with CFP/speaker selection but should not have full event management permissions.

## Proposed feature

Introduce a **Staff-level permission** for events that allows designated contributors to review and manage speaker submissions without granting them full organizer access. The permission model could be similar to the check-in staff access previously available on Bevy for activities such as onboarding and check-ins.

## Scope and behavior

* Allow event organizers to assign **Staff** permissions to specific contributors.
* Staff members should be able to:

* View speaker/CFP submissions.
* Review and evaluate submissions.
* Update the relevant review/status fields where applicable.
* Collaborate on speaker selection.
* Staff members should **not** automatically receive full organizer/admin permissions.
* Permissions should be scoped to the specific event or relevant event workflow.
* The same permission model could potentially be extended to other operational activities such as attendee onboarding and check-ins.

## Motivation

Event organizers frequently rely on volunteers, community members, and external contributors to help review CFP submissions and manage event operations.

Providing a dedicated Staff permission would enable this collaboration while maintaining appropriate access controls, reducing the need to use external tools such as Sessionize solely because contributors cannot access OCG's speaker workflow.