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>

tests/test_feed.py

@@ -9,6 +9,7 @@ from app.main import app
 from app.models.category import Category
 from app.models.channel import Channel
 from app.models.channel_category import channel_categories
+from app.models.download_job import DownloadJob
 from app.models.video import Video
 
 
@@ -109,6 +110,35 @@ def test_feed_filters_uncategorized(client, db_session):
     assert ids == {"vid1", "vid3"}
 
 
+def test_feed_filters_downloaded(client, db_session):
+    _, _, _, videos = _seed(db_session)
+
+    completed = DownloadJob(video_id=videos[0].id, status="completed", media_url="http://x/f0.mp4")
+    failed = DownloadJob(video_id=videos[1].id, status="failed")
+    db_session.add_all([completed, failed])
+    db_session.commit()
+
+    resp = client.get("/api/feed?downloaded=true").json()
+
+    ids = {i["youtube_video_id"] for i in resp["items"]}
+    assert ids == {"vid0"}
+
+
+def test_feed_filters_downloaded_uses_latest_job_per_video(client, db_session):
+    _, _, _, videos = _seed(db_session)
+
+    older = DownloadJob(video_id=videos[0].id, status="completed", media_url="http://x/f0.mp4")
+    db_session.add(older)
+    db_session.commit()
+    newer = DownloadJob(video_id=videos[0].id, status="deleted")
+    db_session.add(newer)
+    db_session.commit()
+
+    resp = client.get("/api/feed?downloaded=true").json()
+
+    assert resp["items"] == []
+
+
 def test_feed_item_shape(client, db_session):
     channel_a, _, category, _ = _seed(db_session)