Alerts - Need alert on Percent of sessions affected by collection of issues not just 1 issue

### Problem Statement

For crash alerts, percentage of sessions affected is more meaningful than raw crash counts, since session volume changes and we want to avoid count-based noise.
For example:
- Low traffic: 1,000 sessions, 10 crashes → 1% → alert should trigger
- Peak traffic: 10,000 sessions, 10 crashes → 0.1% → alert should not trigger

We want this logic to work at both levels:
- Individual crash level (available already)
- Overall crash level
It may also be useful to expose percentage of sessions affected issue count alongside percentage of sessions  affected unique users, for better visibility into user impact.

### Solution Brainstorm

_No response_

### Product Area

Alerts

The issue names the Alerts product area but no files, tests, or entry points. First clarify the alerting design for percentage of sessions across a collection of issues, including whether issue-count and unique-user percentages are required; done criteria are not yet defined.

Written by the indexing model from the issue text.