Fix popular fetch and improve date/view_count coverage

Popular fetch now does a two-phase approach: fast flat-playlist to get
IDs in popularity order, then parallel full metadata fetch (8 workers)
to get real view_count and published_at for each video. Previously
flat-playlist mode returned timestamp/view_count as null.

Enrich task now also backfills published_at and view_count (not just
description). Startup limit 3→50, enrichment sleep 2s→0.5s.

Raise all thread pool sizes to match 8-core machine:
- Discovery search: 5→8 workers
- Graph signal: 4→8 workers
- Popular fetch: 5→8 workers
- Download semaphore default 3→6, cap 10→16

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Channel.jsx

@@ -117,7 +117,7 @@ export default function ChannelPage() {
 
   const popularMut = useMutation({
     mutationFn: () => fetchPopularVideos(id),
-    onSuccess: () => scheduleRefetch(20000),
+    onSuccess: () => scheduleRefetch(35000),
   });
 
   const deepSearchMut = useMutation({