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/native_ingest_common.py

@@ -57,6 +57,41 @@ _TITLE_MAX = 40
 _FORBIDDEN = set('<>:"/\\|?*')
 
 
+# -- shared exception taxonomy --------------------------------------------
+# Every native client raises one of these (platform subclasses keep an
+# isinstance-distinct platform name AND the semantic Auth/Drift class), so the
+# base Ingester._failure_result can map them platform-agnostically.
+
+class NativeIngestError(Exception):
+    """Base for a native-ingest client failure. `status_code` carries the HTTP
+    status when the failure was an HTTP response (None for transport/parse);
+    `retry_after` carries the server's 429 Retry-After hint so the cooldown can
+    match it (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 NativeAuthError(NativeIngestError):
+    """Authentication/authorization failure — expired/missing credential or an
+    insufficient tier. The fix is rotating the credential, NOT updating the
+    ingester. Maps to error_type 'auth_error'."""
+
+
+class NativeDriftError(NativeIngestError):
+    """A response did not match the shape the ingester depends on (JSON:API field
+    set, or scraped HTML structure). Fail loud so the import step flags 'the
+    platform changed' instead of silently importing nothing. Maps to API_DRIFT."""
+
+
 # -- HTTP session ----------------------------------------------------------
 
 def make_session(