refactor(native-ingest): shared exception trio + base _failure_result (#899 DRY 2/3)

DRY pass commit 2. The two adapters re-implemented the same auth→drift→429→404
→http→network mapping in _failure_result; only the exception classes + drift
phrasing differed (divergence-bug risk: a new error_type handled in one and not
the other).

- native_ingest_common gains NativeIngestError / NativeAuthError / NativeDriftError
  (status_code + retry_after on the base). Patreon{API,Auth,Drift}Error and
  SubscribeStar{API,Auth,Drift}Error now subclass them via multiple inheritance,
  keeping their isinstance-distinct platform names.
- Ingester._failure_result (base) does the whole mapping via the shared
  NativeAuthError/NativeDriftError taxonomy + status_code; a new platform gets it
  free. New drift_label kwarg supplies the per-platform API_DRIFT phrasing
  ("Patreon API" / "SubscribeStar markup"), preserving the existing message
  (test asserts "Patreon API changed").
- Both adapters drop their near-identical _failure_result overrides and their now
  -unused DownloadResult/ErrorType/*Auth/*Drift imports.

Verified at every consumer (rule 93/§8b): test_patreon_ingester (auth/drift/429/
404/network) and test_subscribestar_native (_failure_result mapping) both exercise
the base method now. Remaining: ingest_core L1/L3 logging (3/3).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/subscribestar_ingester.py

@@ -21,15 +21,8 @@ from collections.abc import Callable
 from pathlib import Path
 
 from ..models import SubscribeStarFailedMedia, SubscribeStarSeenMedia
-from .gallery_dl import DownloadResult, ErrorType
 from .ingest_core import DEAD_LETTER_THRESHOLD, Ingester
-from .subscribestar_client import (
-    MediaItem,
-    SubscribeStarAPIError,
-    SubscribeStarAuthError,
-    SubscribeStarClient,
-    SubscribeStarDriftError,
-)
+from .subscribestar_client import MediaItem, SubscribeStarAPIError, SubscribeStarClient
 from .subscribestar_downloader import SubscribeStarDownloader
 
 __all__ = [
@@ -96,40 +89,11 @@ class SubscribeStarIngester(Ingester):
             ledger_key=_ledger_key,
             platform="subscribestar",
             error_base=SubscribeStarAPIError,
+            # API_DRIFT message phrasing; the base Ingester._failure_result owns
+            # the auth/drift/HTTP→error_type mapping (shared across platforms).
+            drift_label="SubscribeStar markup",
         )
 
-    # -- failure mapping (SubscribeStar exception taxonomy) ----------------
-
-    def _failure_result(self, exc: Exception, _result) -> DownloadResult:
-        """Map a client-level exception to a loud, typed failed DownloadResult —
-        never a silent zero-download "success". SubscribeStarAuthError and
-        SubscribeStarDriftError both subclass SubscribeStarAPIError, so match them
-        before the generic HTTP/transport fallthrough."""
-        message = str(exc)
-        if isinstance(exc, SubscribeStarAuthError):
-            error_type = ErrorType.AUTH_ERROR
-        elif isinstance(exc, SubscribeStarDriftError):
-            error_type = ErrorType.API_DRIFT
-            message = f"SubscribeStar markup changed — scraper needs update: {message}"
-        else:
-            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("SubscribeStar ingest failed (%s): %s", error_type.value, message)
-        result = _result(
-            success=False, return_code=1,
-            error_type=error_type, error_message=message,
-        )
-        if error_type == ErrorType.RATE_LIMITED:
-            result.retry_after_seconds = getattr(exc, "retry_after", None)
-        return result
-
 
 async def verify_subscribestar_credential(
     url: str,