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/tests/test_youtube_refusals.py

@@ -9,6 +9,7 @@ import pytest
 
 from app.auth import has_read_scope
 from app.models import OAuthToken
+from app.sync.explore import explore_ingest_page
 from app.youtube import client as yt
 from app.youtube.client import YouTubeError, refusal_reason
 
@@ -27,8 +28,8 @@ from app.youtube.client import YouTubeError, refusal_reason
         (YouTubeError("No refresh token; user must re-authenticate"), "reconnect"),
         (YouTubeError("m", reason="transport"), "unavailable"),
         (YouTubeError("m", reason="backendError", path="subscriptions"), "unavailable"),
-        # A playlist deleted on YouTube: neither a retry nor a reconnect helps (it said "reconnect").
-        (YouTubeError("m", reason="playlistNotFound", path="playlists"), "gone"),
+        # A missing playlist the user never named (a channel's uploads): nothing they can act on.
+        (YouTubeError("m", reason="playlistNotFound", path="playlistItems"), "unavailable"),
         (YouTubeError("m", reason="emptyAnswer", path="playlistItems"), "unavailable"),
     ],
 )
@@ -36,6 +37,36 @@ def test_a_failure_is_sorted_by_what_the_user_can_do(exc, reason):
     assert refusal_reason(exc) == reason
 
 
+def test_a_missing_playlist_is_gone_only_on_a_route_about_that_playlist():
+    # A playlist deleted on YouTube: neither a retry nor a reconnect helps (it said "reconnect").
+    # Said only where the user picked that one list — anywhere else "this playlist no longer
+    # exists" names nothing on their screen (E32 review round 2).
+    exc = YouTubeError("m", reason="playlistNotFound", path="playlists")
+    assert refusal_reason(exc, playlist=True) == "gone"
+    assert refusal_reason(exc) == "unavailable"
+    assert refusal_reason(YouTubeError("m", reason="quotaExceeded"), playlist=True) == "quota"
+
+
+def test_a_channel_without_uploads_explores_as_an_empty_page():
+    # YouTube answers a channel's uploads playlist as not found when the channel has no uploads.
+    class _Yt:
+        def get_playlist_items(self, *_):
+            raise YouTubeError("m", reason="playlistNotFound", path="playlistItems")
+
+    channel = SimpleNamespace(id="UCempty", uploads_playlist_id="UUempty")
+    assert explore_ingest_page(None, _Yt(), channel) == {"ingested": 0, "next_page_token": None}
+
+
+def test_any_other_explore_failure_still_raises():
+    class _Yt:
+        def get_playlist_items(self, *_):
+            raise YouTubeError("m", reason="quotaExceeded", path="playlistItems")
+
+    channel = SimpleNamespace(id="UCx", uploads_playlist_id="UUx")
+    with pytest.raises(YouTubeError):
+        explore_ingest_page(None, _Yt(), channel)
+
+
 @pytest.mark.parametrize(
     ("route", "target"),
     [