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/patreon_ingester.py

@@ -51,6 +51,7 @@ from .gallery_dl import DownloadResult, ErrorType
 from .patreon_client import (
     MediaItem,
     PatreonAPIError,
+    PatreonAuthError,
     PatreonClient,
     PatreonDriftError,
 )
@@ -242,7 +243,9 @@ class PatreonIngester:
                     break
             else:
                 reached_bottom = True
-        except (PatreonDriftError, PatreonAPIError) as exc:
+        except PatreonAPIError as exc:
+            # Base of PatreonAuthError + PatreonDriftError — catches every
+            # client-level failure; _failure_result maps it to a typed error.
             return self._failure_result(exc, _result)
 
         if errors:
@@ -294,24 +297,35 @@ class PatreonIngester:
     def _failure_result(self, exc: Exception, _result) -> DownloadResult:
         """Map a client-level exception to a loud, typed failed DownloadResult.
 
-        Step 3 keeps this deliberately coarse — auth-ish drift vs. everything
-        else — so the integration is correct end-to-end. Step 4 (drift detection
-        + error categorization) refines the typed-error mapping and adds the
-        contract test. Either way we NEVER return a silent zero-download
-        "success": the whole point of the native ingester is to fail red when
-        Patreon's API shape or our auth changes.
+        We NEVER return a silent zero-download "success" — the whole point of the
+        native ingester is to fail RED when Patreon's API shape or our auth
+        changes. The typed mapping lets FailingSourcesCard render the right chip
+        and tells the operator what to do:
+          - PatreonAuthError  → AUTH_ERROR  (rotate cookies)
+          - PatreonDriftError → API_DRIFT   (ingester field-set/parser needs update)
+          - HTTP 429 / 404    → RATE_LIMITED / NOT_FOUND
+          - other HTTP status → HTTP_ERROR;  transport failure → NETWORK_ERROR
+
+        PatreonAuthError and PatreonDriftError both subclass PatreonAPIError, so
+        they must be matched before the generic HTTP/transport fallthrough.
         """
         message = str(exc)
-        lowered = message.lower()
-        if isinstance(exc, PatreonDriftError):
-            if "login" in lowered or "challenge" in lowered or "auth" in lowered:
-                error_type = ErrorType.AUTH_ERROR
-            else:
-                error_type = ErrorType.UNKNOWN_ERROR
+        if isinstance(exc, PatreonAuthError):
+            error_type = ErrorType.AUTH_ERROR
+        elif isinstance(exc, PatreonDriftError):
+            error_type = ErrorType.API_DRIFT
             message = f"Patreon API changed — ingester needs update: {message}"
-        else:
-            error_type = ErrorType.NETWORK_ERROR
-        log.warning("Patreon ingest failed: %s", message)
+        else:  # generic PatreonAPIError: HTTP non-2xx (status_code set) or transport
+            status = getattr(exc, "status_code", None)
+            if status == 429:
+                error_type = ErrorType.RATE_LIMITED
+            elif status == 404:
+                error_type = ErrorType.NOT_FOUND
+            elif status is not None:
+                error_type = ErrorType.HTTP_ERROR
+            else:
+                error_type = ErrorType.NETWORK_ERROR
+        log.warning("Patreon ingest failed (%s): %s", error_type.value, message)
         return _result(
             success=False, return_code=1,
             error_type=error_type, error_message=message,