feat(patreon): within-pass transient retry for media GETs — #8 (was overstated as done)

Honest completion of roadmap #8. Previously a NON-429 media failure (a
connection reset, a timeout, a truncated stream, a 5xx) was an immediate
terminal "error" for the pass — only retried on the NEXT walk. Now
_fetch_to_file retries TRANSIENT failures in-place with backoff (transport
blips incl. mid-download, 429 honoring Retry-After, and 5xx; up to 3 tries),
while PERMANENT failures (404 gone / 403 forbidden) fail fast straight to the
error → dead-letter path — re-fetching them is pointless. This makes the
transient-vs-permanent split explicit instead of leaning on the next-tick
cycle. (#1's 429 backoff + #7's dead-letter covered most of #8's value; this
is the missing in-pass transient piece I'd loosely marked "folded".)

Tests: a connection blip / a 5xx is retried then succeeds; a 404 errors with
NO retry; an exhausted transient becomes a terminal error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_patreon_downloader.py

@@ -56,15 +56,19 @@ class _FakeSession:
 
 
 class _SeqSession:
-    """Returns the queued responses in order (for retry tests)."""
+    """Returns the queued items in order (for retry tests). An Exception item is
+    RAISED by get() instead of returned (simulates a transport error)."""
 
-    def __init__(self, responses: list[_FakeResponse]):
+    def __init__(self, responses):
         self._responses = list(responses)
         self.calls: list[str] = []
 
     def get(self, url, stream=False, timeout=None):
         self.calls.append(url)
-        return self._responses.pop(0)
+        item = self._responses.pop(0)
+        if isinstance(item, Exception):
+            raise item
+        return item
 
 
 def _post(post_id="1001", title="My Post Title", published="2026-05-01T12:00:00Z"):
@@ -314,3 +318,54 @@ def test_media_429_retried_then_succeeds(tmp_path, monkeypatch):
     assert outcomes[0].status == "downloaded"
     assert 4.0 in slept  # backed off before the retry
     assert len(session.calls) == 2
+
+
+def _dl(tmp_path, session, monkeypatch):
+    monkeypatch.setattr(pd_mod.time, "sleep", lambda s: None)
+    return PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False, session=session,
+    )
+
+
+def test_transient_transport_error_retried_within_pass(tmp_path, monkeypatch):
+    """plan #705 #8: a connection blip on the GET is retried in-place, not a
+    terminal error that waits for the next walk."""
+    session = _SeqSession([
+        requests.ConnectionError("connection reset"),
+        _FakeResponse(_PNG_BYTES, status_code=200),
+    ])
+    dl = _dl(tmp_path, session, monkeypatch)
+    outcomes = dl.download_post(_post(), [_img("a.png")], "artist-x")
+    assert outcomes[0].status == "downloaded"
+    assert len(session.calls) == 2
+
+
+def test_5xx_retried_within_pass(tmp_path, monkeypatch):
+    session = _SeqSession([
+        _FakeResponse(b"", status_code=503),
+        _FakeResponse(_PNG_BYTES, status_code=200),
+    ])
+    dl = _dl(tmp_path, session, monkeypatch)
+    outcomes = dl.download_post(_post(), [_img("a.png")], "artist-x")
+    assert outcomes[0].status == "downloaded"
+    assert len(session.calls) == 2
+
+
+def test_404_fails_fast_no_retry(tmp_path, monkeypatch):
+    """A permanent 4xx (gone/forbidden) is NOT retried — it errors immediately
+    (and the ingester's dead-letter ledger takes over across walks)."""
+    session = _SeqSession([
+        _FakeResponse(b"", status_code=404),
+        _FakeResponse(_PNG_BYTES, status_code=200),  # would succeed if it retried
+    ])
+    dl = _dl(tmp_path, session, monkeypatch)
+    outcomes = dl.download_post(_post(), [_img("a.png")], "artist-x")
+    assert outcomes[0].status == "error"
+    assert len(session.calls) == 1  # no retry on a permanent failure
+
+
+def test_exhausted_transient_becomes_error(tmp_path, monkeypatch):
+    session = _SeqSession([requests.Timeout("t")] * 10)  # always times out
+    dl = _dl(tmp_path, session, monkeypatch)
+    outcomes = dl.download_post(_post(), [_img("a.png")], "artist-x")
+    assert outcomes[0].status == "error"