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/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.