Slack integration drops segment create/update/delete events

## Summary
The Grafana integration posts annotations for segment create/update/delete events. The Slack integration sends nothing for the same events. Feature flag changes and segment overrides on a feature do reach Slack. We need to add standalone segment changes.

## Steps to reproduce
1. Configure both the Grafana and Slack integrations on the same project, with the Slack config mapped to an environment.
2. Create, update, or delete a segment.
3. Compare the two integrations.

**Observed:** Grafana receives an annotation for the segment change. Slack receives nothing. A feature toggle and a segment override on a feature both post to Slack.

**Expected:** Segment create/update/delete events post to the configured Slack channel, matching Grafana.

## Related issues (not duplicates)
- **#7983** Slack doesn't notify on identity flag overrides.
- **#4090** Add links to the audit log and entities in Slack messages.

Start by locating the Slack and Grafana integration event handlers and compare how segment create, update, and delete events are routed. Reproduce the issue with both integrations configured, then verify that each standalone segment change produces a Slack message in the configured channel.

Written by the indexing model from the issue text.