Add lazy comment fetching to watch page

- VideoComment model (video_id, author, text, likes, is_pinned, published_at)
- fetch_video_comments() in ytdlp.py: top 20 comments, no reply threads,
  sorted pinned-first then by likes
- GET /videos/by-yt/{id}/comments — returns cached comments instantly
- POST /videos/by-yt/{id}/comments/refresh — fetches from YouTube, stores, returns
- Watch page: CommentsSection shows "Load comments" button when uncached,
  renders comments with author/likes once loaded; Refresh link to re-fetch

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

backend/routers/videos.py

@@ -638,6 +638,72 @@ def delete_bookmark(
         db.commit()
 
 
+@router.get("/by-yt/{youtube_video_id}/comments")
+def get_comments(
+    youtube_video_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    from ..models import VideoComment
+    video = db.query(Video).filter_by(youtube_video_id=youtube_video_id).first()
+    if not video:
+        return []
+    comments = (
+        db.query(VideoComment)
+        .filter_by(video_id=video.id)
+        .order_by(VideoComment.is_pinned.desc(), VideoComment.likes.desc())
+        .all()
+    )
+    return [
+        {
+            "author": c.author,
+            "text": c.text,
+            "likes": c.likes,
+            "is_pinned": c.is_pinned,
+            "published_at": c.published_at,
+        }
+        for c in comments
+    ]
+
+
+@router.post("/by-yt/{youtube_video_id}/comments/refresh")
+def refresh_comments(
+    youtube_video_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    from ..models import VideoComment
+    video = db.query(Video).filter_by(youtube_video_id=youtube_video_id).first()
+    if not video:
+        raise HTTPException(status_code=404, detail="Video not found")
+
+    # Clear existing and re-fetch
+    db.query(VideoComment).filter_by(video_id=video.id).delete()
+    db.commit()
+
+    fetched = ytdlp.fetch_video_comments(youtube_video_id)
+    for c in fetched:
+        db.add(VideoComment(video_id=video.id, **c))
+    db.commit()
+
+    comments = (
+        db.query(VideoComment)
+        .filter_by(video_id=video.id)
+        .order_by(VideoComment.is_pinned.desc(), VideoComment.likes.desc())
+        .all()
+    )
+    return [
+        {
+            "author": c.author,
+            "text": c.text,
+            "likes": c.likes,
+            "is_pinned": c.is_pinned,
+            "published_at": c.published_at,
+        }
+        for c in comments
+    ]
+
+
 @router.get("/queue", response_model=list[VideoDetail])
 def queued_videos(
     db: Session = Depends(get_db),