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/youtube/client.py

@@ -117,17 +117,28 @@ _DEAD_GRANT = "invalid_grant"
 YouTubeRefusal = Literal["quota", "reconnect", "unavailable", "gone"]
 
 
-def refusal_reason(exc: YouTubeError) -> YouTubeRefusal:
+def is_playlist_gone(exc: YouTubeError) -> bool:
+    """Whether YouTube answered that the playlist read or written does not exist. Only the caller
+    knows which playlist that was: a user's list deleted on YouTube, or a channel's uploads list,
+    which YouTube also answers so for a channel with no uploads at all."""
+    return "playlistNotFound" in exc.reason.split(",")
+
+
+def refusal_reason(exc: YouTubeError, *, playlist: bool = False) -> YouTubeRefusal:
     """Sort a failure by what the user can do about it. A dead grant (Google refused the refresh
     with `invalid_grant`, which also drops the grant — see `_drop_dead_grant`) or no usable token at
     all (raised before any endpoint answered, so with no path and no transport reason) means the
     account must be reconnected; retrying cannot help. Any other token-endpoint failure — a 5xx, a
     429, a misconfigured client — leaves the grant as it was: reconnecting would not fix it, and
-    the account would still read as connected, with no connect button to press."""
+    the account would still read as connected, with no connect button to press.
+
+    `playlist`: the action was on ONE playlist the user picked, so a missing playlist is that list,
+    gone (deleted on YouTube) — neither a retry nor a reconnect helps. Anywhere else the playlist
+    is one the user never named (a channel's uploads, one list of a bulk pull), and "this playlist
+    no longer exists" would describe nothing on their screen (E32 review round 2)."""
     if is_quota_refusal(exc):
         return "quota"
-    if exc.reason == "playlistNotFound":
-        # The list itself is gone (deleted on YouTube): neither a retry nor a reconnect helps.
+    if playlist and is_playlist_gone(exc):
         return "gone"
     if exc.path == "oauth2/token":
         return "reconnect" if exc.reason == _DEAD_GRANT else "unavailable"