[Change Requests] - Endpoint to update the activation buffer days

Description

The activation buffer is a number of days stored on the organization as activationBufferDays. It controls how far a work package's activation start date can drift from its scheduled date before the change is treated as significant. The column exists with a default, but there's currently no way to change the value from the app; this ticket adds the backend endpoint to update it. (A separate frontend ticket builds the input that calls this.)

Task
Add a service method, controller method, and route that let an admin update the organization's activationBufferDays, with validation.

Acceptance Criteria

• OrganizationsService.setActivationBufferDays(value, submitter, organization) updates the org's activationBufferDays and returns the updated org.
• Only admins can call it
• Invalid values are rejected: negative numbers and non-integers.
• A controller method and route expose it, following the same shape as other organization settings.