Segment-edit-triggered membership count refresh #7470

Scope

Add a Segment.post_save(version_of_id == id) signal handler that enqueues a new task, e.g. refresh_segment_counts(segment_id) to update the membership counts for that specific segment across all environments in the project. The Alpha daily refresh stays as a safety net; the segment-edit path means a saved segment refreshes its counts within minutes, not the next day.

Debounce: if a refresh for the same project is already pending or running, the signal is a no-op.

Acceptance criteria

• Saving a canonical segment edit enqueues exactly one task.
• Saving a non-canonical segment revision (version_of_id != id) does not enqueue.
• Saving when the org has the FoF flag off does not enqueue.
• Existing daily fallback continues to refresh projects whose segments haven't been edited.