Hide unsubscribed channels from the channels list, add a Saved tab

- GET /api/channels called with subscribed=true from the Channels page, so
  a channel disappears from the list as soon as it's unsubscribed instead
  of lingering with a badge
- GET /api/feed gains a `downloaded` filter (this was already anticipated
  in the original TZ's feed query params but left unimplemented until
  download_jobs existed) -- matches on each video's *latest* job only, so
  a redownload/delete history doesn't leave stale matches
- New /saved route: same category filtering, video grid and download
  actions as the main feed, scoped to downloaded videos only

2 new backend tests (76 total).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/Feed.tsx

@@ -53,6 +53,7 @@ function Feed() {
       <header>
         <h1>Лента</h1>
         <div className="actions">
+          <Link to="/saved">Сохранённые</Link>
           <Link to="/channels">Каналы</Link>
           <Link to="/categories">Категории</Link>
           <button onClick={() => syncMutation.mutate()} disabled={running}>