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/services/ytdlp.py

@@ -382,6 +382,46 @@ def fetch_channel_links(channel_id: str) -> list[str]:
     return list(channel_ids)
 
 
+def fetch_video_comments(youtube_video_id: str, max_comments: int = 20) -> list[dict]:
+    """Fetch top comments for a single video. Returns empty list on failure."""
+    url = f"https://www.youtube.com/watch?v={youtube_video_id}"
+    args = [
+        "yt-dlp", url,
+        "--dump-json",
+        "--write-comments",
+        "--extractor-args", f"youtube:max_comments={max_comments},max_comment_depth=1",
+        "--no-download",
+        "--no-playlist",
+        "--quiet",
+        *_cookie_args(),
+    ]
+    stdout, _, code = _run(args, timeout=60)
+    if not stdout.strip():
+        return []
+    try:
+        info = json.loads(stdout.strip())
+    except json.JSONDecodeError:
+        return []
+
+    result = []
+    for c in (info.get("comments") or []):
+        if c.get("parent") not in (None, "root"):
+            continue  # skip replies
+        ts = c.get("timestamp")
+        published_at = datetime.utcfromtimestamp(ts) if ts else None
+        result.append({
+            "youtube_comment_id": c.get("id"),
+            "author": c.get("author"),
+            "text": c.get("text"),
+            "likes": c.get("like_count") or 0,
+            "is_pinned": bool(c.get("is_pinned")),
+            "published_at": published_at,
+        })
+    # Sort pinned first, then by likes
+    result.sort(key=lambda c: (not c["is_pinned"], -(c["likes"] or 0)))
+    return result[:max_comments]
+
+
 QUALITY_FORMATS = {
     "best":  "bestvideo[ext=mp4][vcodec^=avc1]+bestaudio[ext=m4a]/bestvideo[ext=mp4]+bestaudio[ext=m4a]/22/18/bestvideo+bestaudio/best",
     "2160p": "bestvideo[ext=mp4][height<=2160]+bestaudio[ext=m4a]/bestvideo[height<=2160]+bestaudio/best[height<=2160]",