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>

backend/app/services/gallery_dl.py

@@ -40,6 +40,12 @@ class ErrorType(StrEnum):
     HTTP_ERROR = "http_error"
     UNSUPPORTED_URL = "unsupported_url"
     VALIDATION_FAILED = "validation_failed"
+    # Native Patreon ingester only (plan #697): a response parsed as JSON but
+    # didn't match the JSON:API shape the ingester depends on (missing `data`,
+    # media lacking `file_name`/`url`, etc.). Distinct from AUTH_ERROR — the fix
+    # is updating the ingester's field-set/parser, not rotating credentials. The
+    # contract test guards against silently shipping this.
+    API_DRIFT = "api_drift"
     # Run made real progress (downloaded ≥1 file) but did not finish in the
     # subprocess budget. Distinct from UNKNOWN_ERROR — the downstream status
     # mapping classifies this as "ok" because the next tick continues.