Channel-level overview showing subscribers, CTR, and today's impressions.

Aggregates the latest subscriber count per channel from ChannelImpressionSnapshot, joins with EstimatedCTR, and sums ArticleImpressionSnapshot rows for today.

Response shape: { "channels": [ { "channel_id": <int|null>, "channel_name": <str>, "subscribers": <int>, "ctr": <float|null>, "impressions_today": <int>, "source": <str>, # dominant source for today } ] }