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/Channels.tsx

@@ -16,6 +16,7 @@ function Channels() {
     queryKey: ['channels', filter],
     queryFn: () =>
       listChannels({
+        subscribed: true,
         categoryId: typeof filter === 'number' ? filter : undefined,
         uncategorized: filter === 'uncategorized',
       }),
@@ -58,6 +59,7 @@ function Channels() {
         <h1>Каналы</h1>
         <div className="actions">
           <Link to="/">Лента</Link>
+          <Link to="/saved">Сохранённые</Link>
           <Link to="/categories">Категории</Link>
           <button onClick={() => syncMutation.mutate()} disabled={running}>
             {running ? 'Синхронизация...' : 'Обновить сейчас'}