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_client.py

@@ -43,6 +43,9 @@ import requests
 from ..utils.paths import filehash_from_url
 from .native_ingest_common import (
     _MAX_429_RETRIES,
+    NativeAuthError,
+    NativeDriftError,
+    NativeIngestError,
     basename_from_url,
     make_session,
     retry_after_seconds,
@@ -76,30 +79,18 @@ _NEXT_PAGE_RE = re.compile(
 _LOGIN_MARKERS = ("/session/new", 'data-role="sign_in"', "age_confirmation_warning")
 
 
-class SubscribeStarAPIError(Exception):
-    """Base for native SubscribeStar client failures. `status_code` carries the
-    HTTP status when the failure was an HTTP response (None for transport/parse),
-    so the ingester can map it to a DownloadResult.error_type."""
-
-    def __init__(
-        self,
-        message: str,
-        *,
-        status_code: int | None = None,
-        retry_after: float | None = None,
-    ):
-        super().__init__(message)
-        self.status_code = status_code
-        self.retry_after = retry_after
+class SubscribeStarAPIError(NativeIngestError):
+    """Base for native SubscribeStar client failures. status_code / retry_after
+    are inherited from NativeIngestError."""
 
 
-class SubscribeStarAuthError(SubscribeStarAPIError):
+class SubscribeStarAuthError(SubscribeStarAPIError, NativeAuthError):
     """Auth/authorization failure — expired cookies, missing age cookie, or an
     HTML login/age wall served where the feed was expected. Fix = rotate the
     credential, not update the scraper. Maps to error_type 'auth_error'."""
 
 
-class SubscribeStarDriftError(SubscribeStarAPIError):
+class SubscribeStarDriftError(SubscribeStarAPIError, NativeDriftError):
     """The feed HTML did not match the structure we scrape (no recognizable post
     blocks AND no known empty-feed state). The scrape analog of API drift — fail
     loud so the import step flags 'SubscribeStar changed its markup' instead of