fix(youtube): "this playlist no longer exists" only where the user picked that list

playlistNotFound was mapped to "gone" everywhere: exploring a channel without uploads (its
uploads list answers not found) and a list deleted mid bulk pull both said a playlist no
longer exists. The reason is now "gone" only on single-playlist routes; explore reads it
as an empty back-catalogue, and the bulk pull skips that list and syncs the rest.

backend/app/sync/explore.py

@@ -25,7 +25,7 @@ from app.models import (
 )
 from app.sync.videos import insert_stubs, stub_from_playlist_item, apply_video_details
 from app.utils import now_utc as _now
-from app.youtube.client import YouTubeClient
+from app.youtube.client import YouTubeClient, YouTubeError, is_playlist_gone
 
 log = logs.get_logger(__name__)
 
@@ -81,7 +81,15 @@ def explore_ingest_page(
     the organic deep backfill) is never touched."""
     if not channel.uploads_playlist_id:
         return {"ingested": 0, "next_page_token": None}
-    data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
+    try:
+        data = yt.get_playlist_items(channel.uploads_playlist_id, page_token)
+    except YouTubeError as exc:
+        # A channel with no uploads has an uploads id that YouTube answers as not found: an empty
+        # back-catalogue, not a failure (and not a "playlist gone" — the user picked a channel).
+        if not is_playlist_gone(exc):
+            raise
+        log.info("explore ingest channel=%s has no uploads playlist", channel.id)
+        return {"ingested": 0, "next_page_token": None}
     stubs: list[dict] = []
     for item in data.get("items", []):
         stub = stub_from_playlist_item(item, channel.id)