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