Add Manual Segment Rebuild Controls with Status Information

Hello Mautic Team,

Description:
In Mautic 6, segment membership is currently rebuilt exclusively by a cron job (commonly every 5 minutes). This has several drawbacks:
* Users cannot control when the rebuild actually occurs, leading to unnecessary waiting.
* The time-based rebuilds run even when no one is actively using the system, wasting server and database resources.
* At scale, this inefficiency is amplified, increasing both system load and energy consumption unnecessarily.

Proposed Feature:
Introduce Manual Segment Rebuild Controls in the Mautic UI to give users greater flexibility, transparency, and system efficiency.

Key Enhancements:

1. Manual Rebuild Buttons:
* Add a “Rebuild Now” button to the segment detail view.
* Provide two options:
** Rebuild this segment only
** Rebuild all segments

2. Status & Visibility Enhancements:
* Display the timestamp of the last successful rebuild (per segment when viewing a segment, and global when rebuilding all).
* If another team member has already triggered a manual rebuild, display:
* The queued rebuild requests OR
* The next cron job run time
* Apply permissions so only authorised users can run manual rebuilds.

3. Efficiency & Scale Benefits:
* Improves system response times, especially for teams running Mautic at scale with many segments and contacts.
* Reduces wasted compute cycles by enabling targeted (per-segment) rebuilds instead of forcing full rebuilds every 5 minutes.
* Supports environmentally conscious usage by avoiding unnecessary resource consumption.

Example User Flow:
* A marketer edits a segment.
* Instead of waiting for the next cron job, they click “Rebuild This Segment Now”.
* Alternatively, a privileged marketer or admin can click “Rebuild All Segments” if a bulk refresh is required.
* The interface shows:
** Last rebuild completed: 4 minutes ago
** Next scheduled rebuild: in 1 minute OR Manual rebuild queued by \[User]

Impact:

This feature will:
* Give users control over when rebuilds occur.
* Provide transparency into the rebuild process.
* Reduce wasted server/database usage.
* Improve responsiveness and scalability for large instances.
* Support sustainable operation by minimizing unnecessary power consumption.

Thank you for considering this improvement and for your continued work on making Mautic more flexible and user-friendly.