Overhaul For You feed ranking and freshness

Ranking improvements:
- Wider candidate pool (4x limit) with ±12pt score perturbation so
  same-score videos shuffle differently each load
- Recent channel engagement signal: channels watched in past 30 days
  get a +4pts/watch boost
- Bail penalty: -25pts for videos started but abandoned before 20%
- Impression penalty: -3pts per prior feed appearance (capped at 10),
  so repeatedly-skipped videos sink naturally
- rn cap raised to 5 for more candidates; Python-side sampling picks top limit

Feed UX:
- Reshuffle button now available on For You (ranked) mode, not just Explore
- shuffleKey now always included in query key (not just random mode)
- Ranked mode staleTime reduced from 10min to 90s

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

frontend/src/pages/Home.jsx

@@ -56,9 +56,9 @@ export default function Home() {
   };
 
   const { data: feedData = [], isLoading: loadingFeed } = useQuery({
-    queryKey: ["home-feed", mode, page, hideWatched, duration, mode === "random" ? shuffleKey : 0],
+    queryKey: ["home-feed", mode, page, hideWatched, duration, shuffleKey],
     queryFn: () => homeFeed(page, PAGE_SIZE, mode, duration).then((r) => r.data),
-    staleTime: 10 * 60_000,
+    staleTime: mode === "ranked" ? 90_000 : 10 * 60_000,
     placeholderData: (prev) => prev,
   });
 
@@ -195,9 +195,13 @@ export default function Home() {
           {mode === "chronological" && (
             <p className="text-xs text-zinc-600">All videos from channels you follow, newest first.</p>
           )}
-          {mode === "random" && (
+          {(mode === "ranked" || mode === "random") && (
             <div className="flex items-center justify-between">
-              <p className="text-xs text-zinc-600">Random from your discovery pool — no weighting, no ranking.</p>
+              <p className="text-xs text-zinc-600">
+                {mode === "ranked"
+                  ? "Ranked by your taste — reshuffles show a fresh mix."
+                  : "Random from your discovery pool — no weighting, no ranking."}
+              </p>
               <button
                 onClick={handleReshuffle}
                 className="flex items-center gap-1.5 text-xs text-zinc-500 hover:text-zinc-300 transition-colors shrink-0 ml-4"