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/App.tsx

@@ -3,6 +3,7 @@ import { Route, Routes } from 'react-router-dom'
 import { getAuthStatus } from './api/client'
 import Connect from './pages/Connect'
 import Feed from './pages/Feed'
+import Saved from './pages/Saved'
 import Channels from './pages/Channels'
 import ChannelVideos from './pages/ChannelVideos'
 import VideoPage from './pages/VideoPage'
@@ -43,6 +44,7 @@ function App() {
     <div className="app">
       <Routes>
         <Route path="/" element={<Feed />} />
+        <Route path="/saved" element={<Saved />} />
         <Route path="/channels" element={<Channels />} />
         <Route path="/channels/:channelId/videos" element={<ChannelVideos />} />
         <Route path="/video/:youtubeVideoId" element={<VideoPage />} />