fix(quota): keep the demo account off three quota-burning paths (C-3.6)

The demo account could spend real YouTube quota where the human-only guard was
missing (the channel_detail enrichment already had it):
- channels PATCH: an immediate run_recent_backfill when deep_requested is turned on;
- discovery: the channel-detail enrich on the Discover tab;
- feed get_video_detail: the off-catalog videos.list lookup for a video we don't
  store (the DB-hit path stays free; demo now gets a 404 for unknown videos).

backend/app/routes/feed.py

@@ -737,6 +737,11 @@ def get_video_detail(
             "duration_seconds": v.duration_seconds,
         }
 
+    # The DB path above is free; the off-catalog YouTube lookup below spends a quota unit, so the
+    # demo account can't reach it — it just sees "unknown" for a video we don't already store.
+    if user.is_demo:
+        raise HTTPException(status_code=404, detail="Unknown video")
+
     try:
         with quota.attribute(user.id, quota.QuotaAction.VIDEOS_LOOKUP), YouTubeClient(db, user) as yt:
             items = yt.get_videos([video_id])