Popular tab: rank by real view_count, drop broken ?sort=p URL

yt-dlp's own test suite marks channel sort as 'Query for sorting no
longer works' — YouTube blocked it. New approach: fetch view_count for
up to 200 indexed videos in parallel (8 workers, prioritising those
missing counts), then Popular tab sorts by view_count DESC WHERE
view_count IS NOT NULL. Accurate for any channel once enrichment runs.
Frontend refetch wait raised to 60s to cover ~200 parallel fetches.

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(8000),
+    onSuccess: () => scheduleRefetch(60000),
   });
 
   const deepSearchMut = useMutation({
@@ -495,7 +495,7 @@ export default function ChannelPage() {
             {activeQ
               ? `No indexed videos match "${activeQ}"`
               : tab === "popular"
-              ? "No popular videos fetched yet."
+              ? "No view counts yet — click \"Fetch popular\" to rank indexed videos by views."
               : "No videos indexed yet."}
           </p>
           {activeQ ? (