Option to edit the time frame of Slack notification snaps

### Problem Statement

When customers get their Slack notification for an alert, the snapshot shows them over a month of data.

### Solution Brainstorm

It's be far more helpful for them to have the snapshot image only show a time range of their choice.

### Product Area

Settings - Integrations

The issue identifies Settings - Integrations and Slack notification snapshots as the relevant entry points. No files or tests are named, so first locate the existing Slack snapshot flow and its coverage. Done means an agreed time-range setting controls the period shown in the snapshot.

Written by the indexing model from the issue text.