Discover - Filter Issues by Time Relative to App Start

### Summary
Add the ability to filter issues/events based on how much time has elapsed since app start. This would allow users to either isolate or exclude issues that occur within a specific time window after app launch.

### Problem
Currently, there is no way to directly filter issues by time-since-app-start. The closest workaround is filtering by Transactions that include a cold start measurement (e.g. `MainActivity`, `SystemEnabledSettingsActivity`), but this is an imprecise proxy and doesn't give users control over a specific time window.

### Use Case
A user wants to confirm whether a given issue is correlated with app startup — for example, to validate that it occurs predominantly within the first 10 seconds of the app launching. Today this is not possible without significant manual investigation.

### Proposed Solution
Expose a filter in the Issues view (and ideally in Discover) that allows filtering by time elapsed since app start, e.g.:
- **Show only** issues that occurred within X seconds of app start / transaction
- **Exclude** issues that occurred within X seconds of app start / transaction

This could be surfaced as a tag or measurement (e.g. `time_since_app_start`) captured as part of the event payload, or derived from existing span/transaction timing data.

### Current Workaround
Filter by transactions known to contain cold start measurements. This is limited because:
- It requires users to know which transactions correspond to app start
- It doesn't allow filtering by a specific time window (e.g. first 10 seconds)
- Not all startup-related events are captured under a startup transaction

- User confirmed: even a rough "first 10 seconds" filter would be sufficient for their use case

### Solution Brainstorm

_No response_

### Product Area

Discover

The issue names the Issues view, Discover, transaction timing, and event payloads, but no files, tests, or entry points. Start by tracing how Issues and Discover filters use event or transaction timing data; done means users can include or exclude issues within a configurable interval after app start.

Written by the indexing model from the issue text.