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_discord_client.py

@@ -414,7 +414,7 @@ def test_the_request_profile_is_gallery_dls():
 # -- the adapter ---------------------------------------------------------------
 
 def test_the_adapter_authenticates_with_the_token_and_keys_by_identity(tmp_path):
-    from backend.app.services.discord_ingester import DiscordIngester, _ledger_key
+    from backend.app.services.discord_ingester import DiscordIngester
 
     ing = DiscordIngester(tmp_path, None, session_factory=None, auth_token="tok")
     assert ing.client._session.headers["Authorization"] == "tok"
@@ -423,7 +423,7 @@ def test_the_adapter_authenticates_with_the_token_and_keys_by_identity(tmp_path)
     [media] = DiscordClient.extract_media(
         _msg(9, attachments=[{"id": "300", "url": "https://cdn/a/1.png"}])
     )
-    assert _ledger_key(media) == "9:300"
+    assert ing._ledger_key(media) == "9:300"
 
 
 # -- the poster picker (#4491) -------------------------------------------------