- Rewrite list_channels to run exactly 4 SQL queries regardless of channel
count: channel rows, aggregated video stats (GROUP BY), new-video counts,
and latest video (derived-table JOIN replaces per-row correlated subquery)
- Remove dead _CHANNEL_STATS_SELECT (orphaned after the rewrite)
- Fix upload_frequency_days: use pre-computed date_span_days from vstats
instead of a broken per-channel db.execute() call
- Restrict new_counts query to id_csv so it uses idx_videos_channel_indexed
- markChannelsSeen: optimistic setQueryData instead of invalidateQueries,
eliminating a full channel-list re-fetch on every Following page visit
- DownloadIndicator idle poll: 10s → 30s (no need to hit DB when idle)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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