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/app/youtube/client.py

@@ -93,6 +93,14 @@ def _upstream_failure(
 # The `mine` reads' answer for a Google account that has no YouTube channel.
 _NO_CHANNEL = frozenset({"channelNotFound"})
 
+# The reasons that mean the project's quota is spent: every further call today gets the same answer.
+_QUOTA_REASONS = frozenset({"quotaExceeded", "dailyLimitExceeded"})
+
+
+def is_quota_refusal(exc: YouTubeError) -> bool:
+    """Whether the API refused for want of quota. `reason` can list several codes, comma-joined."""
+    return any(r in _QUOTA_REASONS for r in exc.reason.split(","))
+
 
 def _chunks(items: list, size: int) -> Iterator[list]:
     for i in range(0, len(items), size):