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/models.py

@@ -165,6 +165,20 @@ class UserTagAffinity(Base):
     updated_at = Column(DateTime, default=datetime.utcnow)
 
 
+class VideoComment(Base):
+    __tablename__ = "video_comments"
+
+    id = Column(Integer, primary_key=True, index=True)
+    video_id = Column(Integer, ForeignKey("videos.id", ondelete="CASCADE"), nullable=False, index=True)
+    youtube_comment_id = Column(String)
+    author = Column(String)
+    text = Column(Text)
+    likes = Column(Integer, default=0)
+    is_pinned = Column(Boolean, default=False)
+    published_at = Column(DateTime)
+    fetched_at = Column(DateTime, default=datetime.utcnow)
+
+
 class Collection(Base):
     __tablename__ = "collections"