fix(feed): repair 2 bugs found in the Phase 2 #2 review

BUG-1 (search.py, high): the scrape search source logged one VIDEOS_SEARCH
quota event PER continuation page inside the paging loop, but actions_today
counts events — so a single user search that pages N times consumed N against
search_daily_limit_per_user (its docstring even warns it only works for
once-per-action logging). Log exactly once per request after the loop instead;
the API source already logs once via record_usage (it never auto-pages).

BUG-2 (Feed.tsx, medium): the optimistic-override reset effect keyed on
query.dataUpdatedAt also fired on fetchNextPage (infinite scroll bumps
dataUpdatedAt while page 1 is unchanged), so a just-hidden card flashed back
mid-scroll. Guard with a page-count ref: only reset on a real refetch, not an
append. The two override-reset effects legitimately differ now (resolves the
would-be C-F1 "identical effects" cleanup).

tsc green, ruff clean, localdev boots healthy.

frontend/src/components/Feed.tsx

@@ -184,9 +184,18 @@ export default function Feed({
     setOverrides({});
     setSavedOverrides({});
   }, [filters]);
+  // ...but NOT on infinite-scroll appends: fetchNextPage also bumps dataUpdatedAt while page 1's
+  // rows are unchanged, so clearing overrides there would make a just-hidden card flash back.
+  // Only a real refetch (page count doesn't grow) is authoritative.
+  const pagesLenRef = useRef(0);
   useEffect(() => {
-    setOverrides({});
-    setSavedOverrides({});
+    const len = query.data?.pages?.length ?? 0;
+    const appended = len > pagesLenRef.current;
+    pagesLenRef.current = len;
+    if (!appended) {
+      setOverrides({});
+      setSavedOverrides({});
+    }
   }, [query.dataUpdatedAt]);
 
   const countQuery = useQuery({