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_contract.py

@@ -0,0 +1,72 @@
+"""Patreon API contract test (native ingester build step 4).
+
+The drift tripwire the plan calls for: if the field-set we SEND or the parser we
+resolve responses against drifts from what real Patreon returns, this build goes
+RED — instead of the ingester silently importing nothing in production (the exact
+failure mode the native ingester exists to escape).
+
+Two halves:
+  - the recorded `/api/posts` fixture must still parse end-to-end (no drift), and
+  - the request params must still carry every field the parser depends on (the
+    fixture already has the data, so trimming the request would NOT trip the
+    parse half — this half guards the request shape directly).
+
+Pure: no network, no DB.
+"""
+
+import json
+from pathlib import Path
+
+from backend.app.services.patreon_client import MediaItem, PatreonClient
+
+_FIXTURE = Path(__file__).parent / "fixtures" / "patreon_posts_page1.json"
+
+# Pinned media total across the 4 fixture posts: 1001→2 (gallery, image_large
+# cover deduped), 1002→1 (attachment+postfile collapse), 1003→1 (inline content
+# img), 1004→1 (video postfile). A parser change that drops or doubles media
+# trips this.
+_EXPECTED_MEDIA_TOTAL = 5
+_KNOWN_KINDS = {"images", "image_large", "attachments", "postfile", "content"}
+
+
+def test_recorded_fixture_parses_end_to_end():
+    response = json.loads(_FIXTURE.read_text())
+    client = PatreonClient(cookies_path=None)
+
+    client._validate_response(response)  # top-level shape must hold
+    index = client._transform(response)
+
+    total = 0
+    for post in response["data"]:
+        items = client.extract_media(post, index)  # must not raise drift
+        for m in items:
+            assert isinstance(m, MediaItem)
+            assert m.url, f"empty url in post {post['id']}"
+            assert m.filename, f"empty filename in post {post['id']}"
+            assert m.kind in _KNOWN_KINDS, f"unknown kind {m.kind!r}"
+            assert m.post_id == post["id"]
+        total += len(items)
+
+    assert total == _EXPECTED_MEDIA_TOTAL
+
+
+def test_request_field_set_carries_parser_dependencies():
+    """The params we SEND must keep requesting the fields the parser resolves
+    against. Trimming `fields[media]=file_name`, an `include` relationship, or a
+    `fields[post]` key would make real responses parse to nothing — caught here,
+    not by the fixture parse above."""
+    params = PatreonClient(cookies_path=None)._params("5555", None)
+
+    media_fields = params["fields[media]"]
+    assert "file_name" in media_fields
+    assert "image_urls" in media_fields or "download_url" in media_fields
+
+    includes = params["include"]
+    for rel in ("images", "attachments_media", "media"):
+        assert rel in includes, f"missing include relationship {rel}"
+
+    post_fields = params["fields[post]"]
+    for field in ("content", "post_file", "image"):
+        assert field in post_fields, f"missing post field {field}"
+
+    assert params["filter[campaign_id]"] == "5555"