Reject with reason, keep post in Backoffice as "Rejected" and notify user

Description: As a moderator, I want to reject an idea from the moderation queue and keep it in the backoffice with a "Rejected" status, while automatically notifying the submitter with the specific rejection reason, so that the idea is not publicly visible but remains accessible for audit, reporting, and potential follow-up. Problem: Currently, using Reject in moderation removes the post from the admin/backoffice, which prevents internal tracking, auditing, and re-evaluation. If moderators avoid rejecting to retain visibility, they cannot provide a specific rejection reason or notify the submitter. This creates data loss, inconsistent workflows, and poor communication with contributors. Benefit: Maintain an internal record of rejected items (status, reason, moderator, timestamp) for transparency and compliance. Improve contributor experience with clear, actionable rejection messages. Enable consistent moderation UX and internal reporting/filtering on rejection reasons. Allow future reconsideration of rejected ideas without re-submission.