feat(patreon): ingester rate-limit resilience — #703 step 1

A single 429 mid-walk used to fail the run → RATE_LIMITED → platform-wide
cooldown → every Patreon source dark ("testing dead"). The native path also
ignored the operator's existing politeness setting. Fixed both:

- Pacing (avoid 429s): honor download_rate_limit_seconds (gallery-dl's
  `rate_limit`, read off self.gdl) as a pre-download sleep on real media
  downloads only (skips don't pace); pace /api/posts page fetches with the
  per-source sleep_request override, defaulting to max(0.5, rate_limit/4) —
  the same API-pacing default gallery-dl used for `sleep-request`.
- 429 backoff (ride out transient limits): PatreonClient._fetch retries a
  429 with backoff (honor Retry-After, else exponential 2·2^(n-1), capped
  30s, ≤3 tries); only a PERSISTENT 429 propagates as terminal
  RATE_LIMITED. Light 2-retry on a media-GET 429 too.

Threaded via PatreonIngester(rate_limit=, request_sleep=) →
PatreonClient/PatreonDownloader; download_service sources them. Injected
test client/downloader are unaffected (carry their own pacing).

Tests mock time.sleep (no real sleeping): retry-then-success, persistent
429 raises after N, Retry-After honored, request_sleep paces, media pacing
per real download, skips don't pace, media 429 retried.

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

tests/test_patreon_client.py

@@ -10,12 +10,14 @@ from pathlib import Path
 
 import pytest
 
+from backend.app.services import patreon_client as pc_mod
 from backend.app.services.patreon_client import (
     MediaItem,
     PatreonAPIError,
     PatreonAuthError,
     PatreonClient,
     PatreonDriftError,
+    _retry_after_seconds,
     parse_cursor_from_url,
 )
 
@@ -192,10 +194,11 @@ def test_media_referenced_but_absent_raises_drift(client):
 
 
 class _FakeResp:
-    def __init__(self, status_code, *, json_data=None, raise_json=False):
+    def __init__(self, status_code, *, json_data=None, raise_json=False, headers=None):
         self.status_code = status_code
         self._json_data = json_data
         self._raise_json = raise_json
+        self.headers = headers or {}
 
     def json(self):
         if self._raise_json:
@@ -275,3 +278,62 @@ def test_verify_auth_inconclusive_on_network(monkeypatch):
     monkeypatch.setattr(client._session, "get", _raise)
     ok, _msg = client.verify_auth("5555")
     assert ok is None
+
+
+# -- 429 backoff + pacing (plan #703) --------------------------------------
+
+
+def test_retry_after_seconds_honors_numeric_header():
+    assert _retry_after_seconds(_FakeResp(429, headers={"Retry-After": "7"}), 1) == 7.0
+
+
+def test_retry_after_seconds_exponential_without_header():
+    resp = _FakeResp(429)
+    assert _retry_after_seconds(resp, 1) == 2.0
+    assert _retry_after_seconds(resp, 2) == 4.0
+    assert _retry_after_seconds(resp, 3) == 8.0
+
+
+def test_retry_after_seconds_caps():
+    assert _retry_after_seconds(_FakeResp(429), 10) == 30.0
+    assert _retry_after_seconds(_FakeResp(429, headers={"Retry-After": "999"}), 1) == 30.0
+
+
+def _client_with_sequence(monkeypatch, responses):
+    """A client whose session yields `responses` in order; time.sleep stubbed so
+    backoff never really sleeps. Returns (client, slept_list)."""
+    client = PatreonClient(cookies_path=None)
+    it = iter(responses)
+    monkeypatch.setattr(client._session, "get", lambda *a, **k: next(it))
+    slept: list[float] = []
+    monkeypatch.setattr(pc_mod.time, "sleep", lambda s: slept.append(s))
+    return client, slept
+
+
+def test_fetch_retries_then_succeeds_on_429(monkeypatch):
+    client, slept = _client_with_sequence(monkeypatch, [
+        _FakeResp(429, headers={"Retry-After": "5"}),
+        _FakeResp(200, json_data={"data": []}),
+    ])
+    assert client._fetch("5555", None) == {"data": []}
+    assert 5.0 in slept  # backed off once before the retry
+
+
+def test_fetch_persistent_429_raises_after_retries(monkeypatch):
+    client, _slept = _client_with_sequence(monkeypatch, [_FakeResp(429)] * 10)
+    with pytest.raises(PatreonAPIError) as ei:
+        client._fetch("5555", None)
+    assert ei.value.status_code == 429
+    assert not isinstance(ei.value, PatreonAuthError)
+
+
+def test_request_sleep_paces_before_fetch(monkeypatch):
+    client = PatreonClient(cookies_path=None, request_sleep=1.5)
+    monkeypatch.setattr(
+        client._session, "get",
+        lambda *a, **k: _FakeResp(200, json_data={"data": []}),
+    )
+    slept: list[float] = []
+    monkeypatch.setattr(pc_mod.time, "sleep", lambda s: slept.append(s))
+    client._fetch("5555", None)
+    assert slept == [1.5]