fix(sync): only a refusal about the channel spends its fallback window

A failed connection or token step answered nothing about the channel, yet stamped it: one proxy blip
during an RSS blackout delayed every due channel by a whole window. YouTubeError now names the
endpoint that answered; a failure no endpoint gave, or a spent quota, ends the round and stamps
nothing.

backend/app/sync/runner.py

@@ -197,9 +197,10 @@ def run_feed_fallback(db: Session, channel_ids: list[str]) -> dict[str, int]:
     uploads playlist is 1 unit, so a whole day of total outage costs 12 units per channel at the
     default 2 hours; it yields to the backfill reserve like every other background read. The clock is
     `last_rss_at` or `feed_fallback_at`, whichever is later, so a channel is read at most once per
-    window whether its feed came back in between or not. A FAILED read stamps the clock too: a
-    terminated channel's uploads playlist answers 404 for good, and without the stamp it would be
-    charged again on every 20-minute poll instead of once per window."""
+    window whether its feed came back in between or not. A read the API REFUSED stamps the clock
+    too: a terminated channel's uploads playlist answers 404 for good, and without the stamp it
+    would be charged again on every 20-minute poll instead of once per window. A failure that is not
+    about the channel (no connection, no token, the quota spent) stamps nothing and ends the round."""
     hours = sysconfig.get_int(db, "rss_fallback_hours")
     if hours <= 0 or not channel_ids:
         return {"read": 0, "new": 0, "failed": 0}
@@ -235,10 +236,11 @@ def run_feed_fallback(db: Session, channel_ids: list[str]) -> dict[str, int]:
             # already logged the call's own failure (`youtube.api.error`).
             except YouTubeError as exc:
                 failed += 1
-                _stamp_fallback_attempt(db, channel.id)
-                # Out of quota upstream: every further channel would be charged the same refusal.
-                if is_quota_refusal(exc):
+                # Not an answer about THIS channel — no connection, no token, no quota: every
+                # further channel would fail the same way, and none of them has used its window.
+                if exc.path != "playlistItems" or is_quota_refusal(exc):
                     break
+                _stamp_fallback_attempt(db, channel.id)
                 continue
             rows = [
                 s

backend/app/youtube/client.py

@@ -26,11 +26,15 @@ class YouTubeError(Exception):
 
     `reason` is the API's own error reason code (`quotaExceeded`, `subscriptionDuplicate`,
     `videoNotFound`, …) when the body carried one. Branch on THAT, never on the message: the
-    message is prose, it changes under us, and it is deliberately the poorer of the two here."""
+    message is prose, it changes under us, and it is deliberately the poorer of the two here.
 
-    def __init__(self, message: str, *, reason: str = ""):
+    `path` is the API endpoint that ANSWERED with the failure (`playlistItems`); empty when no
+    endpoint did — the request never got through (`transport`) or the token step refused first."""
+
+    def __init__(self, message: str, *, reason: str = "", path: str = ""):
         super().__init__(message)
         self.reason = reason
+        self.path = path
 
 
 def _error_reason(resp: httpx.Response) -> str:
@@ -87,7 +91,9 @@ def _upstream_failure(
             # log-pii-ok: only under this user's own Teljes consent, which is what it is for
             extra=logs.event("youtube.api.body", body=resp.text[:500]),
         )
-    return YouTubeError(f"{method} {path} -> {resp.status_code}: {reason}", reason=reason)
+    return YouTubeError(
+        f"{method} {path} -> {resp.status_code}: {reason}", reason=reason, path=path
+    )
 
 
 # The `mine` reads' answer for a Google account that has no YouTube channel.

backend/tests/test_feed_fallback.py

@@ -30,7 +30,10 @@ 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])
+            reason = self.failing[playlist_id]
+            # A request that never got through has no endpoint that answered.
+            path = "" if reason == "transport" else "playlistItems"
+            raise YouTubeError("refused", reason=reason, path=path)
         cid = "UC" + playlist_id[2:]
         return {
             "items": [
@@ -134,10 +137,14 @@ def test_a_read_that_fails_waits_the_window_out_too(db, calls):
     assert calls == ["UUgone"]
 
 
-def test_out_of_quota_upstream_stops_the_round(db, calls):
+@pytest.mark.parametrize("reason", ["quotaExceeded", "transport"])
+def test_a_failure_not_about_the_channel_ends_the_round_and_costs_no_window(db, calls, reason):
+    # Out of quota, or no connection: every further channel would fail the same way, and a channel
+    # that was never answered about keeps its turn for the next poll.
     _channel(db, "UCa")
     _channel(db, "UCb")
-    _FakeYt.failing["UUa"] = "quotaExceeded"
+    _FakeYt.failing["UUa"] = reason
 
     assert runner.run_feed_fallback(db, ["UCa", "UCb"])["failed"] == 1
     assert calls == ["UUa"]
+    assert db.get(Channel, "UCa").feed_fallback_at is None