fix(dataflows): honour Reddit Retry-After: 0 and jitter 429 backoff

- a valid Retry-After: 0 means retry at once but was treated as absent
  (`or 5.0`) and waited 5s; honour it exactly now
- jitter our own headerless fallback and the inter-subreddit pacing so several
  analyses sharing an IP don't retry in lockstep and re-collide on the limit;
  keep the single-retry ceiling (more retries can't fix an exhausted IP budget) #1193

tests/test_reddit_fallback.py

@@ -147,6 +147,26 @@ class TestRss429Backoff:
             reddit._fetch_subreddit_rss("NVDA", "stocks", 5, 5.0)
         slept.assert_called_once_with(12.0)
 
+    def test_retry_after_zero_is_honoured_not_treated_as_absent(self):
+        # A valid "Retry-After: 0" means retry at once; it must not fall through
+        # to the fallback wait (the earlier `or 5.0` bug turned 0 into 5s).
+        err = HTTPError("url", 429, "Too Many Requests", {"Retry-After": "0"}, None)
+        with patch.object(reddit, "urlopen", side_effect=[err, _atom_resp()]), \
+             patch.object(reddit.time, "sleep") as slept:
+            reddit._fetch_subreddit_rss("NVDA", "stocks", 5, 5.0)
+        slept.assert_called_once_with(0.0)
+
+    def test_headerless_429_fallback_is_jittered(self):
+        # No Retry-After -> our own ~5s fallback, jittered so concurrent runs
+        # don't retry in lockstep (kept within a tight band).
+        err = HTTPError("url", 429, "Too Many Requests", {}, None)
+        with patch.object(reddit, "urlopen", side_effect=[err, _atom_resp()]), \
+             patch.object(reddit.time, "sleep") as slept:
+            reddit._fetch_subreddit_rss("NVDA", "stocks", 5, 5.0)
+        slept.assert_called_once()
+        (wait,), _ = slept.call_args
+        assert 4.0 <= wait <= 6.0  # 5s +/-20% jitter
+
 
 @pytest.mark.unit
 class TestChunkedTransferErrorsHandled: