fix(sync): don't try a timed-out feed again

A request that hung for the full timeout says the way out is hanging, not
that the endpoint is flaky; three of them per channel stretched a poll over
a dead egress to about three times its interval.

backend/tests/test_sync_guards.py

@@ -282,6 +282,14 @@ class TestRssErrorVsEmpty:
         assert rss.fetch_channel_feed("UC_blip")
         assert len(tries) == 2
 
+    def test_a_timeout_is_not_tried_again(self, monkeypatch):
+        # A hung request already cost the full timeout; retrying it would triple a poll over a
+        # dead egress.
+        tries, waits = self._answers(monkeypatch, httpx.ReadTimeout("hung"), 200)
+        with pytest.raises(rss.RssError) as err:
+            rss.fetch_channel_feed("UC_hung")
+        assert (len(tries), waits, err.value.reason) == (1, [], "transport")
+
     def test_a_feed_that_keeps_failing_raises_its_last_failure(self, monkeypatch):
         tries, _ = self._answers(monkeypatch, 500, 404, 404)
         with pytest.raises(rss.RssError) as err: