fix(alerts): improve Slack API rate limiting for large workspaces (#35622)

docs/docs/configuration/alerts-reports.mdx

@@ -65,6 +65,22 @@ To send alerts and reports to Slack channels, you need to create a new Slack App
 
 Note: when you configure an alert or a report, the Slack channel list takes channel names without the leading '#' e.g. use `alerts` instead of `#alerts`.
 
+#### Large Slack Workspaces (10k+ channels)
+
+For workspaces with many channels, fetching the complete channel list can take several minutes and may encounter Slack API rate limits. Add the following to your `superset_config.py`:
+
+```python
+from datetime import timedelta
+
+# Increase cache timeout to reduce API calls
+# Default: 1 day (86400 seconds)
+SLACK_CACHE_TIMEOUT = int(timedelta(days=2).total_seconds())
+
+# Increase retry count for rate limit errors
+# Default: 2
+SLACK_API_RATE_LIMIT_RETRY_COUNT = 5
+```
+
 ### Kubernetes-specific
 
 - You must have a `celery beat` pod running. If you're using the chart included in the GitHub repository under [helm/superset](https://github.com/apache/superset/tree/master/helm/superset), you need to put `supersetCeleryBeat.enabled = true` in your values override.