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>

tests/test_subscribestar_native.py

@@ -13,8 +13,8 @@ from pathlib import Path
 
 import requests
 
-from backend.app.services.gallery_dl import DownloadResult, ErrorType
-from backend.app.services.native_ingest_common import post_dir_name
+from backend.app.services.download_result import DownloadResult, ErrorType
+from backend.app.services.native_ingest_common import ledger_key, post_dir_name
 from backend.app.services.subscribestar_client import (
     MediaItem,
     SubscribeStarAPIError,
@@ -26,10 +26,7 @@ from backend.app.services.subscribestar_client import (
     _split_creator_url,
 )
 from backend.app.services.subscribestar_downloader import SubscribeStarDownloader
-from backend.app.services.subscribestar_ingester import (
-    SubscribeStarIngester,
-    _ledger_key,
-)
+from backend.app.services.subscribestar_ingester import SubscribeStarIngester
 from backend.app.utils.sidecar import find_sidecar
 
 _PNG_HEAD = b"\x89PNG\r\n\x1a\n"
@@ -489,12 +486,12 @@ def test_skip_seen_does_not_download(tmp_path):
 # -- ingester ----------------------------------------------------------------
 
 def test_ledger_key_prefers_filehash_then_post_media():
-    assert _ledger_key(_img("a.jpg")) == "111:9001"  # no filehash → post:media
+    assert ledger_key(_img("a.jpg")) == "111:9001"  # no filehash → post:media
     hashed = MediaItem(
         url="u", filename="a.jpg", kind="image", filehash="deadbeef",
         post_id="111", media_id="9001",
     )
-    assert _ledger_key(hashed) == "deadbeef"
+    assert ledger_key(hashed) == "deadbeef"
 
 
 def _ingester(tmp_path):