Add reports and subscriptions for test results and regressions

## Context

Multiple users described workflows where they want KernelCI to proactively deliver relevant information rather than requiring them to repeatedly inspect the dashboard.

Examples include following a particular test or collection of tests, receiving regression reports, seeing failures grouped by kernel version and architecture, and receiving concise summaries by email.

Email is particularly useful because reports can be reviewed asynchronously and even when dashboard access is inconvenient or unavailable.

## Goal

Design a subscription/reporting mechanism that lets maintainers define what KernelCI results they care about and receive useful summaries when relevant results become available.

Points to discuss
• What users can subscribe to: tests, collections, trees, branches, architectures, etc.
• Event-driven notifications versus periodic digests.
• Whether notifications should focus on regressions rather than every execution.
• Email format and content.
• Integration with automatic baseline comparison.
• Whether email should be the first delivery mechanism while keeping the model extensible to other channels.
• Relationship with the existing registered-issue notification mechanism.

The issue names no files, tests, or entry points. Start by reviewing the existing registered-issue notification mechanism and automatic baseline comparison, then define the subscription model, report triggers, email content, and extension points; done means an agreed design that answers the listed discussion points.

Written by the indexing model from the issue text.