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_downloader.py

@@ -10,7 +10,9 @@ from __future__ import annotations
 from pathlib import Path
 
 import pytest
+import requests
 
+from backend.app.services import patreon_downloader as pd_mod
 from backend.app.services.patreon_client import MediaItem
 from backend.app.services.patreon_downloader import (
     MediaOutcome,
@@ -26,10 +28,14 @@ _PNG_BYTES = _PNG_HEAD + (b"\x00" * 16) + _PNG_TAIL
 
 
 class _FakeResponse:
-    def __init__(self, payload: bytes):
+    def __init__(self, payload: bytes, status_code: int = 200, headers=None):
         self._payload = payload
+        self.status_code = status_code
+        self.headers = headers or {}
 
     def raise_for_status(self):
+        if self.status_code >= 400:
+            raise requests.HTTPError(f"HTTP {self.status_code}")
         return None
 
     def iter_content(self, chunk_size=65536):
@@ -49,6 +55,18 @@ class _FakeSession:
         return _FakeResponse(self.payloads.get(url, _PNG_BYTES))
 
 
+class _SeqSession:
+    """Returns the queued responses in order (for retry tests)."""
+
+    def __init__(self, responses: list[_FakeResponse]):
+        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)
+
+
 def _post(post_id="1001", title="My Post Title", published="2026-05-01T12:00:00Z"):
     return {
         "id": post_id,
@@ -250,3 +268,46 @@ def test_media_outcome_shape():
     o = MediaOutcome(media=None, status="downloaded", path=Path("/tmp/x"), error=None)
     assert o.status == "downloaded"
     assert o.path == Path("/tmp/x")
+
+
+# -- pacing + 429 backoff (plan #703) --------------------------------------
+
+
+def test_rate_limit_paces_each_real_download(tmp_path, monkeypatch):
+    slept: list[float] = []
+    monkeypatch.setattr(pd_mod.time, "sleep", lambda s: slept.append(s))
+    dl = PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False,
+        rate_limit=2.0, session=_FakeSession(),
+    )
+    dl.download_post(_post(), [_img("a.png"), _img("b.png")], "artist-x")
+    # One pacing sleep per actual download (two media downloaded).
+    assert slept == [2.0, 2.0]
+
+
+def test_skips_do_not_pace(tmp_path, monkeypatch):
+    slept: list[float] = []
+    monkeypatch.setattr(pd_mod.time, "sleep", lambda s: slept.append(s))
+    dl = PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False,
+        rate_limit=2.0, session=_FakeSession(),
+    )
+    # is_seen → skipped_seen, never reaches the download/pacing path.
+    dl.download_post(_post(), [_img("a.png")], "artist-x", is_seen=lambda m: True)
+    assert slept == []
+
+
+def test_media_429_retried_then_succeeds(tmp_path, monkeypatch):
+    slept: list[float] = []
+    monkeypatch.setattr(pd_mod.time, "sleep", lambda s: slept.append(s))
+    session = _SeqSession([
+        _FakeResponse(b"", status_code=429, headers={"Retry-After": "4"}),
+        _FakeResponse(_PNG_BYTES, status_code=200),
+    ])
+    dl = PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False, session=session,
+    )
+    outcomes = dl.download_post(_post(), [_img("a.png")], "artist-x")
+    assert outcomes[0].status == "downloaded"
+    assert 4.0 in slept  # backed off before the retry
+    assert len(session.calls) == 2