fix(sync): a failed feed-fallback read waits its window out, and spent quota ends the round

A channel whose uploads playlist also failed was never stamped, so a terminated channel was charged
again on every 20-minute poll instead of once per rss_fallback_hours. An upstream quotaExceeded now
stops the round instead of charging every remaining channel the same refusal.

backend/tests/test_feed_fallback.py

@@ -8,12 +8,16 @@ import pytest
 from app import sysconfig
 from app.models import Channel, Video
 from app.sync import runner
+from app.youtube.client import YouTubeError
 
 NOW = datetime.now(timezone.utc)
 _PUB = datetime(2020, 1, 1, tzinfo=timezone.utc)
 
 
 class _FakeYt:
+    # playlist id -> the reason its read fails with (the tests put entries here)
+    failing: dict[str, str] = {}
+
     def __init__(self, calls):
         self.calls = calls
 
@@ -25,6 +29,8 @@ class _FakeYt:
 
     def get_playlist_items(self, playlist_id, page_token=None):
         self.calls.append(playlist_id)
+        if playlist_id in self.failing:
+            raise YouTubeError("refused", reason=self.failing[playlist_id])
         cid = "UC" + playlist_id[2:]
         return {
             "items": [
@@ -42,6 +48,7 @@ def calls(monkeypatch):
     monkeypatch.setattr(runner, "get_service_user", lambda db: object())
     monkeypatch.setattr(runner, "YouTubeClient", lambda db, user: _FakeYt(made))
     monkeypatch.setattr(runner.quota, "backfill_reserve_reached", lambda: False)
+    monkeypatch.setattr(_FakeYt, "failing", {})
     return made
 
 
@@ -113,3 +120,24 @@ def test_it_yields_to_the_backfill_reserve(db, calls, monkeypatch):
 
     assert runner.run_feed_fallback(db, ["UCdark"])["read"] == 0
     assert calls == []
+
+
+def test_a_read_that_fails_waits_the_window_out_too(db, calls):
+    # A terminated channel: its feed 404s and so does its uploads playlist, for good. Without a
+    # stamp it was charged on every 20-minute poll instead of once per window.
+    _channel(db, "UCgone", rss=NOW - timedelta(hours=3))
+    _FakeYt.failing["UUgone"] = "playlistNotFound"
+
+    assert runner.run_feed_fallback(db, ["UCgone"]) == {"read": 0, "new": 0, "failed": 1}
+    assert db.get(Channel, "UCgone").feed_fallback_at is not None
+    assert runner.run_feed_fallback(db, ["UCgone"])["failed"] == 0
+    assert calls == ["UUgone"]
+
+
+def test_out_of_quota_upstream_stops_the_round(db, calls):
+    _channel(db, "UCa")
+    _channel(db, "UCb")
+    _FakeYt.failing["UUa"] = "quotaExceeded"
+
+    assert runner.run_feed_fallback(db, ["UCa", "UCb"])["failed"] == 1
+    assert calls == ["UUa"]