Combined filtering in alerts and widgets

### Problem Statement

Manual filter management creates unnecessary risk and inefficiency in dashboard and alert maintenance.

Currently, updating filters across multiple dashboard components and alerts is a manual, fragmented process. When teams need to modify filter criteria, they must remember to update each individual widget and alert separately. This scattered approach leads to:

- Inconsistent data views when some components get updated while others are overlooked
- Unreliable alerting due to outdated filter configurations in both dashboard alerts and standalone monitoring rules
- Alert fatigue from false positives caused by mismatched filter settings
- Wasted time spent tracking down and manually updating multiple widgets and alerts
- Increased maintenance burden that grows with dashboard complexity and alert volume

The result? Teams lose confidence in both their dashboards and alerting systems, spending more time managing their monitoring tools than actually monitoring their systems effectively.

### Solution Brainstorm

#### Shared Filter Functionality
Create filters once and use them everywhere! Generate custom filters that can be seamlessly shared across multiple widgets, alerts, and dashboard components. This ensures consistency in your data views and saves time by eliminating the need to recreate the same filters repeatedly.

#### Widget-Based Alert Configuration
Set up intelligent alerts directly from your dashboard widgets with just a few clicks. The best part? Your alerts automatically stay in sync – when you update a widget's configuration, any alerts based on that widget are automatically updated too. This keeps your monitoring system consistent and reduces maintenance overhead.

### Product Area

Alerts

The issue does not name files, tests, or entry points. Start by tracing the existing alert and dashboard-widget filter flows in Sentry, then clarify how shared filters and widget-based alerts should interact. Done means filters can be reused across widgets and alerts, with alerts staying synchronized when their source widget changes.

Written by the indexing model from the issue text.