#54 [Admin Panel] Invitation System - Cancel/Resend Invitation

Implement cancel and resend endpoints for invitations. **API Specifications:** **Cancel Invitation:** - Endpoint: DELETE /api/v1/admin/invitations/<invitation_id> - Permission Required: manage_users - Response: {success: true, invitation_id} **Resend Invitation:** - Endpoint: POST /api/v1/admin/invitations/<invitation_id>/resend - Permission Required: manage_users - Request Body: {extend_expiry?: bool} (default: true) - Response: {success: true, invitation_id, new_expires_at} **Requirements:** - Cancel: Mark as cancelled, invalidate token - Resend: Generate new token, send new email - Resend: Optionally extend expiry - Track resend count - Log audit events - Cannot resend accepted invitations **IMPORTANT:** Implement with ultra attention to details and correctness. **BEFORE CLOSING:** Take screenshot showing cancel/resend. Add comment summarizing implementation.