feat(patreon): drift detection + error categorization — build step 4 (plan #697)

Typed, loud failure mapping for the native Patreon ingester so a changed
API shape or expired auth never silently zero-downloads as "success".

- New ErrorType.API_DRIFT (free varchar error_type col → no migration):
  distinct from auth so the operator knows the fix is updating the
  ingester, not rotating cookies.
- patreon_client: PatreonAPIError carries status_code; new PatreonAuthError
  for 401/403 + HTML-login/non-JSON bodies (reclassified from drift —
  expired-session is auth, actionable as "rotate cookies").
- patreon_ingester._failure_result maps: PatreonAuthError→AUTH_ERROR,
  PatreonDriftError→API_DRIFT ("Patreon API changed — ingester needs
  update"), HTTP 429→RATE_LIMITED, 404→NOT_FOUND, other HTTP→HTTP_ERROR,
  transport→NETWORK_ERROR. (429 thus drives the platform cooldown.)
- FailingSourcesCard: api_drift chip (red) + hint.

Contract test (new test_patreon_contract.py): the recorded /api/posts
fixture must parse end-to-end (no drift, 5 media across 4 posts) AND the
request params must still carry every field the parser depends on
(file_name, image_urls/download_url, images/attachments_media/media
includes, content/post_file/image post fields) — a trim of either trips a
red build. Plus client HTTP-status classification tests and ingester
error-type mapping tests.

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

tests/test_patreon_client.py

@@ -12,6 +12,8 @@ import pytest
 
 from backend.app.services.patreon_client import (
     MediaItem,
+    PatreonAPIError,
+    PatreonAuthError,
     PatreonClient,
     PatreonDriftError,
     parse_cursor_from_url,
@@ -184,3 +186,55 @@ def test_media_referenced_but_absent_raises_drift(client):
     }
     with pytest.raises(PatreonDriftError):
         client.extract_media(post, {})
+
+
+# -- HTTP status classification (plan #697 step 4) -------------------------
+
+
+class _FakeResp:
+    def __init__(self, status_code, *, json_data=None, raise_json=False):
+        self.status_code = status_code
+        self._json_data = json_data
+        self._raise_json = raise_json
+
+    def json(self):
+        if self._raise_json:
+            raise ValueError("Expecting value: line 1 column 1")
+        return self._json_data
+
+
+def _client_returning(monkeypatch, resp):
+    client = PatreonClient(cookies_path=None)
+    monkeypatch.setattr(client._session, "get", lambda *a, **k: resp)
+    return client
+
+
+@pytest.mark.parametrize("status", [401, 403])
+def test_fetch_auth_status_raises_auth_error(monkeypatch, status):
+    client = _client_returning(monkeypatch, _FakeResp(status))
+    with pytest.raises(PatreonAuthError) as ei:
+        client._fetch("5555", None)
+    assert ei.value.status_code == status
+
+
+def test_fetch_non_json_body_is_auth_not_drift(monkeypatch):
+    # An HTML login/challenge page (non-JSON) means the session expired — auth,
+    # actionable as "rotate cookies", NOT API drift.
+    client = _client_returning(monkeypatch, _FakeResp(200, raise_json=True))
+    with pytest.raises(PatreonAuthError):
+        client._fetch("5555", None)
+
+
+@pytest.mark.parametrize("status", [404, 429, 500])
+def test_fetch_other_status_raises_api_error_with_code(monkeypatch, status):
+    client = _client_returning(monkeypatch, _FakeResp(status))
+    with pytest.raises(PatreonAPIError) as ei:
+        client._fetch("5555", None)
+    assert ei.value.status_code == status
+    # Not auth — the ingester maps these by status, not to auth_error.
+    assert not isinstance(ei.value, PatreonAuthError)
+
+
+def test_fetch_ok_returns_payload(monkeypatch):
+    client = _client_returning(monkeypatch, _FakeResp(200, json_data={"data": []}))
+    assert client._fetch("5555", None) == {"data": []}