Add view count to videos

- Video model: view_count column (Integer, nullable)
- ytdlp._normalize_video: extract view_count from yt-dlp info
- _VIDEO_SELECT: include v.view_count in all queries
- VideoDetail schema: view_count field
- Watch page: formatViews() helper, show "X.XM views" in meta row
  alongside date and category

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

backend/models.py

@@ -60,6 +60,7 @@ class Video(Base):
     tags = Column(Text)      # JSON array string
     category = Column(String)
     chapters = Column(Text)  # JSON array of {start_time, end_time, title}
+    view_count = Column(Integer)
 
 
 class UserVideo(Base):