refactor: the native downloaders share one download loop, file record and seen-ledger key; a clean walk marks its credential verified

Patreon, SubscribeStar and Discord each carried their own copy of the same
per-post download loop, file record and seen-ledger key, and every copy was
another place a fix could be missed.

- BaseNativeDownloader runs one post's files: the seen and on-disk skips,
  the mid-post time-box, pacing, validation and the per-file sidecar. Each
  platform overrides only where its layout differs: Discord's channel folder,
  file names and message-id sidecar, and Patreon's Mux video through yt-dlp.
- One MediaItem and one ledger_key in native_ingest_common replace the three
  per-client copies. Keys are unchanged, except that a SubscribeStar file with
  no gallery id is now keyed by its filename instead of an empty id.
- DownloadResult, ErrorType and the run-stats and tier-gated helpers move
  from gallery_dl.py, which now serves only HentaiFoundry, to
  download_result.py. The unused delegator methods on GalleryDLService go.
- The download lock's platform list is the native-ingester list, not a copy.
- A walk that used a credential and finished with no error type stamps that
  credential's last-verified time, so the credentials page shows when the
  account last worked. A tier-limited walk does not count: an expired session
  hides paid posts the same way.
- The extension no longer clears a retired pixiv token on startup.
- Two comments that still described every download as gallery-dl now name
  both backends.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/subscribestar_client.py

@@ -32,7 +32,6 @@ import logging
 import re
 import time
 from collections.abc import Iterator
-from dataclasses import dataclass
 from datetime import datetime
 from html import unescape
 from pathlib import Path
@@ -43,6 +42,7 @@ import requests
 from ..utils.paths import filehash_from_url
 from .native_ingest_common import (
     _MAX_429_RETRIES,
+    MediaItem,
     Membership,
     NativeAuthError,
     NativeDriftError,
@@ -163,24 +163,6 @@ class SubscribeStarDriftError(SubscribeStarAPIError, NativeDriftError):
     silently importing nothing."""
 
 
-@dataclass
-class MediaItem:
-    """One resolved downloadable item belonging to a SubscribeStar post.
-
-    Fields mirror patreon_client.MediaItem (so the downloader is structurally the
-    same) plus `media_id` — the stable per-upload gallery id. SubscribeStar's
-    full-res URL is an opaque `/post_uploads?payload=...` (not content-addressed),
-    so `filehash` is usually None and the ledger keys on `<post_id>:<media_id>`.
-    """
-
-    url: str
-    filename: str
-    kind: str
-    filehash: str | None
-    post_id: str
-    media_id: str
-
-
 def _extr(text: str, start: str, end: str) -> str:
     """Substring between the first `start` and the next `end` after it (gallery-
     dl's `text.extr`); '' when either marker is absent."""