feat(downloads): platform cooldown honors server Retry-After — B1 (plan #708)

Owning the native client means we see the 429 Retry-After header — previously
discarded. PatreonAPIError now carries `retry_after`; on a PERSISTENT page-fetch
429 the client attaches the server's raw Retry-After seconds. New
DownloadResult.retry_after_seconds; patreon_ingester._failure_result sets it on
RATE_LIMITED. download_service._update_source_health passes it to
set_platform_cooldown as `seconds=`, clamped to [60, 3600] (a tiny hint can't
leave the platform effectively un-cooled; a huge one can't strand it for hours);
no hint → the flat 900s default. So a rate-limited platform cools for as long as
the server actually asks, not a fixed guess.

Tests: terminal 429 surfaces retry_after (test_patreon_client); cooldown honors
+ clamps the hint, falls back to default when absent (test_download_service).

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

tests/test_patreon_client.py

@@ -327,6 +327,18 @@ def test_fetch_persistent_429_raises_after_retries(monkeypatch):
     assert not isinstance(ei.value, PatreonAuthError)
 
 
+def test_fetch_persistent_429_surfaces_retry_after(monkeypatch):
+    """plan #708 B1: a terminal 429 carries the server's raw Retry-After seconds
+    (uncapped here — the cooldown clamps) so the platform cooldown matches it."""
+    client, _slept = _client_with_sequence(
+        monkeypatch, [_FakeResp(429, headers={"Retry-After": "120"})] * 10,
+    )
+    with pytest.raises(PatreonAPIError) as ei:
+        client._fetch("5555", None)
+    assert ei.value.status_code == 429
+    assert ei.value.retry_after == 120.0
+
+
 def test_request_sleep_paces_before_fetch(monkeypatch):
     client = PatreonClient(cookies_path=None, request_sleep=1.5)
     monkeypatch.setattr(