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/services/ytdlp.py

@@ -74,6 +74,7 @@ def _normalize_video(info: dict) -> dict:
         "tags": json.dumps(info.get("tags") or []),
         "category": info.get("category") or (info.get("categories") or [None])[0],
         "chapters": json.dumps(chapters) if chapters else None,
+        "view_count": info.get("view_count"),
         "channel": {
             "youtube_channel_id": info.get("channel_id"),
             "name": info.get("channel") or info.get("uploader", ""),