Optimise Following page: 4 aggregated queries, no correlated subqueries

- 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>

frontend/src/components/Layout.jsx

@@ -80,7 +80,7 @@ function DownloadIndicator() {
       const active = (query.state.data ?? []).some(
         (d) => d.status === "pending" || d.status === "downloading"
       );
-      return active ? 1500 : 10_000;
+      return active ? 1500 : 30_000;
     },
   });