Add dateModified search filter for issues

### Problem Statement

When polling for recently changed issues, there's no way to filter by "last modified time". The existing filters (`lastSeen`, `firstSeen`, `age`) track event timestamps, not metadata changes like priority, assignee, or status updates.

This makes it impossible to efficiently query "issues that changed since my last sync" without fetching all issues.

### Solution Brainstorm

Add a `dateModified:>TIMESTAMP` or `updated:>TIMESTAMP` search filter that matches issues where any metadata was modified after the given time.

### Product Area

Issues

Start by locating the existing lastSeen, firstSeen, and age search-filter implementations and the issue-search tests. Determine how metadata changes such as priority, assignee, and status are represented, then define the filter semantics and verify that queries return issues modified after the supplied timestamp.

Written by the indexing model from the issue text.