Track video clicks as engagement signals

- stats: started_count now includes any video opened (last_watched_at set)
  not just ones with saved progress seconds
- VideoPlayer: fires updateProgress immediately on open so even a
  click-and-back sets last_watched_at and counts as a started video

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

backend/routers/stats.py

@@ -140,7 +140,7 @@ def get_stats(
         text("""
             SELECT COUNT(*) AS n FROM user_videos
             WHERE user_id = :uid AND watched = 0
-              AND watch_progress_seconds > 0
+              AND (watch_progress_seconds > 0 OR last_watched_at IS NOT NULL)
         """),
         {"uid": uid},
     ).mappings().first()