Sample videos randomly for view_count enrichment, not newest-first

Previously ORDER BY published_at DESC meant only the newest 200 videos
ever got view counts. Now ORDER BY RANDOM() spreads the 200 slots across
the full channel history — videos without a count are still prioritised,
but among those they're drawn randomly. Each run of Fetch Popular covers
a different slice, converging toward full coverage over time.

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

backend/routers/channels.py

@@ -730,7 +730,7 @@ def _fetch_popular_task(channel_id: int, youtube_channel_id: str):
             text("""
                 SELECT youtube_video_id FROM videos
                 WHERE channel_id = :cid
-                ORDER BY (view_count IS NULL) DESC, published_at DESC NULLS LAST
+                ORDER BY (view_count IS NULL) DESC, RANDOM()
                 LIMIT 200
             """),
             {"cid": channel_id},