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>

backend/app/api/feed.py

@@ -2,13 +2,14 @@ import base64
 from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, Query
-from sqlalchemy import select
+from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
 from app.core.auth_dependency import require_session
 from app.db import get_db
 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
 from app.services.download_jobs import latest_jobs_map
 from app.services.video_presentation import channel_categories_map, serialize_video
@@ -41,6 +42,7 @@ def get_feed(
     category_id: int | None = None,
     uncategorized: bool = False,
     channel_id: int | None = None,
+    downloaded: bool = False,
     limit: int = Query(DEFAULT_LIMIT, ge=1, le=MAX_LIMIT),
     cursor: str | None = None,
     db: Session = Depends(get_db),
@@ -58,6 +60,15 @@ def get_feed(
         )
         query = query.filter(Video.channel_id.in_(channel_ids_in_category))
 
+    if downloaded:
+        # A video may have several download_jobs over time (retries,
+        # redownload after delete); only its most recent one counts.
+        latest_job_ids = select(func.max(DownloadJob.id)).group_by(DownloadJob.video_id)
+        completed_video_ids = select(DownloadJob.video_id).where(
+            DownloadJob.id.in_(latest_job_ids), DownloadJob.status == "completed"
+        )
+        query = query.filter(Video.id.in_(completed_video_ids))
+
     if cursor:
         cursor_published_at, cursor_id = _decode_cursor(cursor)
         query = query.filter(