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/playlists.py

@@ -26,7 +26,7 @@ from app.sync.playlist_merge import merge_plan
 from app.sync.runner import token_users
 from app.sync.subscriptions import blank_listing, refuse_blank_listing, vanished
 from app.sync.videos import apply_video_details, parse_dt
-from app.youtube.client import YouTubeClient, YouTubeError
+from app.youtube.client import YouTubeClient, YouTubeError, is_playlist_gone
 
 log = logs.get_logger(__name__)
 # Cost of a single YouTube write op (playlists/playlistItems insert/update/delete).
@@ -319,6 +319,24 @@ def user_sync_claim(db: Session, user_id: int) -> AbstractContextManager[bool]:
     return _advisory_claim(db, _USER_SYNC_LOCK_NS, user_id)
 
 
+def _mirror_listing(yt: YouTubeClient, ytid: str, count: object, pl_id: int) -> list[str] | None:
+    """A mirror's video ids from YouTube, or None when there is nothing to believe: the list was
+    deleted between the pull's listing and this read (the next pull drops the mirror as vanished —
+    not a reason to fail every other list, E32 review round 2), or YouTube listed no items for a
+    list it counts as non-empty (`blank_listing`)."""
+    try:
+        ids = list(yt.iter_my_playlist_video_ids(ytid))
+    except YouTubeError as exc:
+        if not is_playlist_gone(exc):
+            raise
+        log.info("YouTube playlist of %s vanished mid-pull; skipped", pl_id)
+        return None
+    if blank_listing(ids, count):
+        log.warning("YouTube listed no items for non-empty playlist %s; kept", pl_id)
+        return None
+    return ids
+
+
 def pull_user_playlists(db: Session, user: User) -> dict:
     """The mirror pull itself; the caller holds `user_sync_claim`."""
     synced = 0
@@ -377,9 +395,9 @@ def pull_user_playlists(db: Session, user: User) -> dict:
                     # The user is pushing or re-pulling this very list: it writes the same rows.
                     # The next scheduled run refreshes it.
                     continue
-                ids = list(yt.iter_my_playlist_video_ids(ytid))
-                if blank_listing(ids, p.get("item_count")):
-                    log.warning("YouTube listed no items for non-empty playlist %s; kept", pl.id)
+                ids = _mirror_listing(yt, ytid, p.get("item_count"), pl.id)
+                if ids is None:
+                    db.rollback()  # a mirror this run just created is not kept empty either
                     continue
                 # Ingest first — it commits — so the row lock below is held from the dirty check
                 # to the commit that replaces the items.