Per-channel impression breakdown for a single article.

Returns the latest snapshot per channel (excluding the aggregate 'telegram_total' roll-up row), plus running totals for impressions and clicks split by exact vs. estimated source.

Response shape: { "news_id": <int>, "total_impressions": <int>, "total_clicks": <int>, "source_breakdown": {"exact": <int>, "estimated": <int>}, "channels": [ { "channel_id": <int|null>, "channel_name": <str>, "impressions": <int>, "clicks": <int>, "source": <str>, # 'telegram_api' | 'click_estimated' "snapshot_date": <str>, # YYYY-MM-DD } ], }