Add older content exploration: channel page + home feed Rediscover mode

Channel page:
- "Explore older videos" button fetches 100 videos at a time further back
  in the channel history using yt-dlp --playlist-start/--playlist-end
- "Fetch entire history" still available for full crawl
- Backend: /channels/{id}/explore?page=N endpoint + playlist offset support
  in fetch_channel_metadata(start_video=N)

Home feed:
- New "Rediscover" mode: older unwatched videos (90+ days old) from
  followed channels, randomly sampled then re-ranked by tag affinity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/channels.py

@@ -758,6 +758,54 @@ def index_channel_full(
     return {"detail": "Full index started"}
 
 
+@router.post("/{channel_id}/explore", status_code=status.HTTP_202_ACCEPTED)
+def explore_channel_older(
+    channel_id: int,
+    page: int = 2,
+    background_tasks: BackgroundTasks = None,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Fetch a page of older videos from this channel (page 1 = newest 30, page 2 = next 100, etc.)."""
+    _get_channel_or_404(db, channel_id)
+    start = 1 if page <= 1 else (30 + (page - 2) * 100 + 1)
+    background_tasks.add_task(_index_channel_explore_task, channel_id, current_user.id, start, 100)
+    return {"detail": f"Fetching older videos (page {page})", "start": start}
+
+
+def _index_channel_explore_task(channel_id: int, user_id: int, start_video: int, count: int):
+    from ..database import SessionLocal
+    db = SessionLocal()
+    try:
+        channel = db.query(Channel).filter_by(id=channel_id).first()
+        if not channel:
+            return
+        result = ytdlp.fetch_channel_metadata(channel.youtube_channel_id, max_videos=count, start_video=start_video)
+        if not result:
+            return
+        for vdata in result.get("videos", []):
+            yt_id = vdata.get("youtube_video_id")
+            if not yt_id or not vdata.get("published_at"):
+                continue
+            if not db.query(Video).filter_by(youtube_video_id=yt_id).first():
+                db.add(Video(
+                    youtube_video_id=yt_id,
+                    channel_id=channel.id,
+                    title=vdata.get("title", ""),
+                    description=vdata.get("description"),
+                    thumbnail_url=vdata.get("thumbnail_url"),
+                    duration_seconds=vdata.get("duration_seconds"),
+                    published_at=vdata.get("published_at"),
+                    tags=vdata.get("tags"),
+                    category=vdata.get("category"),
+                ))
+        db.commit()
+    except Exception:
+        db.rollback()
+    finally:
+        db.close()
+
+
 @router.post("/follow-bulk", status_code=200)
 def follow_bulk(
     body: dict,