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/patreon_client.py

@@ -41,6 +41,9 @@ from ..utils.paths import filehash_from_url
 from ..utils.prosemirror import post_body_html
 from .native_ingest_common import (
     _MAX_429_RETRIES,
+    NativeAuthError,
+    NativeDriftError,
+    NativeIngestError,
     basename_from_url,
     make_session,
     retry_after_seconds,
@@ -73,30 +76,12 @@ _FIELDS_CAMPAIGN = "name,url"
 _CONTENT_IMG_RE = re.compile(r"<img\b[^>]*?\bsrc=[\"']([^\"']+)[\"']", re.IGNORECASE)
 
 
-class PatreonAPIError(Exception):
-    """Base for native Patreon client failures.
-
-    `status_code` carries the HTTP status when the failure was an HTTP response
-    (None for transport-level / parse failures), so the ingester can map it to a
-    DownloadResult.error_type (429 → rate_limited, 404 → not_found, …).
-    `retry_after` carries the server's `Retry-After` seconds on a terminal 429, so
-    the platform cooldown can match the server's hint instead of a flat default
-    (plan #708 B1).
-    """
-
-    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 PatreonAPIError(NativeIngestError):
+    """Base for native Patreon client failures. status_code / retry_after are
+    inherited from NativeIngestError (HTTP status; 429 Retry-After hint)."""
 
 
-class PatreonAuthError(PatreonAPIError):
+class PatreonAuthError(PatreonAPIError, NativeAuthError):
     """Authentication / authorization failure — missing or expired session
     cookies, an insufficient pledge tier, or an HTML login/challenge page served
     where JSON was expected. DISTINCT from drift: the fix is rotating the
@@ -104,7 +89,7 @@ class PatreonAuthError(PatreonAPIError):
     """
 
 
-class PatreonDriftError(PatreonAPIError):
+class PatreonDriftError(PatreonAPIError, NativeDriftError):
     """A JSON response did not match the JSON:API shape we depend on.
 
     Raised for: a missing top-level `data` list, `data` not a list, or a media