Customizable email subject for Uptime alerts

### Problem Statement

We use Sentry Uptime monitoring and rely a lot on email alerts for day-to-day operations.
Right now, the email subject for Uptime alerts is fixed, which makes it hard to set up reliable inbox filters—especially when you’re dealing with multiple environments, services, or teams.

Because of this, alerts often end up mixed together in the inbox and require manual sorting, which adds noise and slows down response time for on-call engineers.

### Solution Brainstorm

It would be really helpful if the email subject for Uptime alerts could be customized in some way.

For example:
- Allow a custom subject or subject template
- Support simple variables like project, environment, status

This would make it much easier to filter, route, and prioritize alerts using standard email rules.

### Product Area

Other

The issue names no files, tests, or entry points. Start by locating where Uptime alert emails are assembled and clarify the supported customization model and variables with maintainers. Done should include a defined way to customize subjects and documented behavior for project, environment, and status values.

Written by the indexing model from the issue text.