Implement YearInReviewBuilder to aggregate annual agent performance stats

Implement YearInReviewBuilder to aggregate annual agent performance stats: a task in bird-critic-1.5-full-300 (Harbor dataset). Chatwoot's UI shows agents a personalised "Year in Review" summary — total conversations handled, busiest day, and average first-response time. The data is aggregated…

The task

Chatwoot's UI shows agents a personalised "Year in Review" summary — total conversations handled, busiest day, and average first-response time. The data is aggregated server-side by a new builder class so the frontend can fetch it from a dedicated API endpoint.

Part of kochsnow/bird-critic-1.5-full-300.