fix(sync): a channel with no uploads backfills as done, not as a failure

YouTube answers the uploads list of a channel with no uploads as
playlistNotFound. The recent and deep backfills raised on it, so the channel
was never stamped and was picked again, charged a quota unit and logged an
ERROR on every run. Both now read it as an empty last page, as explore already
does; any other failure still raises.

backend/tests/test_youtube_refusals.py

@@ -8,8 +8,9 @@ import httpx
 import pytest
 
 from app.auth import has_read_scope
-from app.models import OAuthToken
+from app.models import Channel, OAuthToken
 from app.sync.explore import explore_ingest_page
+from app.sync.videos import backfill_channel_deep, backfill_channel_recent
 from app.youtube import client as yt
 from app.youtube.client import YouTubeError, refusal_reason
 
@@ -57,6 +58,52 @@ def test_a_channel_without_uploads_explores_as_an_empty_page():
     assert explore_ingest_page(None, _Yt(), channel) == {"ingested": 0, "next_page_token": None}
 
 
+class _NoUploads:
+    """A client whose every uploads read answers "playlist not found" (a channel with no uploads)."""
+
+    def __init__(self):
+        self.calls = 0
+
+    def get_playlist_items(self, *_):
+        self.calls += 1
+        raise YouTubeError("m", reason="playlistNotFound", path="playlistItems")
+
+
+def test_a_channel_without_uploads_backfills_as_done_and_is_not_charged_again(db):
+    # The recent pass stamped nothing on this answer, so the channel was picked — and charged a
+    # quota unit, with an ERROR traceback — on every backfill run, forever (E32 review round 3).
+    ch = Channel(id="UCempty", title="Empty", uploads_playlist_id="UUempty")
+    db.add(ch)
+    db.commit()
+    yt = _NoUploads()
+    assert backfill_channel_recent(db, yt, ch) == 0
+    db.refresh(ch)
+    assert ch.recent_synced_at is not None and ch.backfill_done and ch.backfill_cursor is None
+    assert backfill_channel_deep(db, yt, ch) == 0
+    assert yt.calls == 1  # the deep pass has nothing left to ask
+
+
+def test_a_deep_backfill_whose_uploads_vanish_ends_instead_of_failing(db):
+    ch = Channel(id="UCgone", title="Gone", uploads_playlist_id="UUgone", backfill_cursor="page2")
+    db.add(ch)
+    db.commit()
+    assert backfill_channel_deep(db, _NoUploads(), ch) == 0
+    db.refresh(ch)
+    assert ch.backfill_done and ch.backfill_cursor is None
+
+
+def test_any_other_backfill_failure_still_raises(db):
+    class _Yt:
+        def get_playlist_items(self, *_):
+            raise YouTubeError("m", reason="quotaExceeded", path="playlistItems")
+
+    ch = Channel(id="UCq", title="Q", uploads_playlist_id="UUq")
+    db.add(ch)
+    db.commit()
+    with pytest.raises(YouTubeError):
+        backfill_channel_recent(db, _Yt(), ch)
+
+
 def test_any_other_explore_failure_still_raises():
     class _Yt:
         def get_playlist_items(self, *_):